Firefox, Thunderbird Lagging? Lag Fix Inside

Martin Brinkmann
Oct 21, 2011
Updated • Jun 26, 2017
Email, Thunderbird

For the last days I have experienced lags and issues both in the Firefox web browser and the Thunderbird email client. With Thunderbird, I experienced lag that resulted in the Thunderbird window not responding for some time.

The window of the email program recovered after some time, so that I could access the program's functionality again.

With Firefox it is another issue. The browser just stops receiving traffic from the Internet as if it is blocked by a firewall or as if the Internet connection died. The only solution here so far was to restart the web browser.

David Bradley discovered a solution that fixed the Thunderbird lag issue, and probably the Firefox no traffic issue as well.

David mentioned frequent stutter or lag in the browser leading to the "not responding" message that can last for a few moments. This was the same behavior that I experienced in Thunderbird.

Fix lags in Thunderbird

The solution or fix here is to assign one specific core to the Firefox, or in my case Thunderbird, process. This obviously only works if a multi-core cpu is available.

Users who just want to test the setting can assign a specific core by opening the Windows Task Manager with Ctrl-Shift-Esc, right-clicking the firefox.exe or thunderbird.exe process and selecting Affinity from the context menu.

firefox process affinity

There they need to select a specific cpu core or remove one from the listing to test the fix. Please note that this is only a temporary solution. A system state change, e.g. a reboot, standby or hibernation resets the affinity settings that you have selected.

A permanent solution is to create a new program shortcut and set the cpu affinity this way.

The command for Firefox is this one:

C:\Windows\System32\cmd.exe /c start "C:\Program Files\Mozilla Firefox\" /affinity x firefox.exe

where x is the cpu core number. (cpu 0 = 1, cpu 1 = 2, cpu 2 = 4, cpu 3 = 8 and so on).

For Thunderbird the command is

C:\Windows\System32\cmd.exe /c start "C:\Program Files (x86)\Miramar 3.3 Alpha 1\thunderbird.exe" /affinity x thunderbird.exe

The easiest way is to copy the shortcut from the start menu or to use Windows Explorer to open the folder where the program is installed. There you can right-click firefox.exe or thunderbird.exe and select to create a new shortcut.

It is likely that we will see an official fix in the coming days / weeks. You can then simply delete the shortcut or modify it again so that all cpu cores are used by the application. It is unlikely that this change has an effect on the program's performance.

Windows XP users cannot make use of the command. The suggestion here is to use the freeware RunFirst which sets the process affinity automatically to one core.

Has this fixed your issue as well? Let me know in the comments. (thanks David, via)

Firefox, Thunderbird Lagging? Lag Fix Inside
Article Name
Firefox, Thunderbird Lagging? Lag Fix Inside
If Firefox or Thunderbird lag, you may want to try assigning one specific cpu core to the processes as it may fix the lags that you experience.
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  1. JMGG said on January 19, 2012 at 8:25 am

    You said that Outlook isn’t your main email client, so which is your main one?

    1. BalaC said on January 19, 2012 at 9:42 am

      I think its thunderbird

    2. Martin Brinkmann said on January 19, 2012 at 10:15 am

      It is Mozilla Thunderbird.

  2. Salaam said on September 24, 2012 at 9:52 pm

    Awesome! This actually solved my problem… what a stupid bug.

  3. Claud said on December 19, 2012 at 2:08 am

    If this is the same bug that I’ve encountered, there may be another fix: (1) hover over open Outlook item in Taskbar, cursor up to hover over Outlook window item, and right-click; (2) this should give you Restore / Move / Size / Minimize / Maximize — choose Move or Size; (3) use your cursor keys, going arbitrarily N/S/E/W, to try to move or size the Outlook window back into view. Basically, the app behaves as though it were open in a 0x0 window, or at a location that’s offscreen, and this will frequently work to resize and/or move the window. Don’t forget to close while resized/moved, so that Outlook remembers the size/position for next time.

    1. Lynda said on February 12, 2013 at 3:37 pm

      THANK YOU Claude!!! I could get the main window to launch but could not get any other message window to show on the desktop. You are my hero!!!!

    2. Chad said on November 20, 2018 at 4:24 pm

      Solved my issue! 6 years later and this is still problem…

    3. Ivan X said on January 21, 2021 at 4:50 pm

      Fantastic. Thank you. Size did the trick.

  4. Andrew said on October 26, 2013 at 7:06 am

    This solved my Outlook problem, too. Thank you. :)

  5. Charles said on December 7, 2013 at 7:23 pm

    Thank you so much, this started happening to me today and was causing big problems. You are a life saver, I hope I can help you in some way some day.

  6. garth said on November 7, 2014 at 7:13 pm

    You are a god – thank you!

  7. Faisal said on February 9, 2015 at 10:09 am

    thanks a lot…. work like charm.. :-)

  8. Simon said on March 24, 2015 at 11:36 pm

    Yah…thanks Claude. I’ve been having the same problem and tried all the suggestions…your solution was the answer. It had resized itself to a 0/0 box. Cheers

  9. Olu said on April 14, 2015 at 1:35 pm

    Excellent post. This had me baffled even trying to accurately describe the problem. This fixed it for me.
    Thank you

  10. Coenig said on July 23, 2015 at 7:36 am

    Thanks a lot for the article. Don’t know why it happenend, don’t know how it got fixed, but it was really annoying and now it works :-)

  11. Fali said on January 20, 2016 at 4:19 pm

    Thanks a lot. I was facing this issue from past 3 week. I tried everything but no resolution. The issue was happening intermittently and mainly when I was changing the display of screen ( as i use 2 monitors). The only option i had was to do system restore. But thanks to you.

    1. MIki said on January 10, 2019 at 11:54 am

      I’ve been tried to sole this problem for 12hours. Your comment about changing the display of screen helped me a lot!! Thanks!!

  12. Christina said on January 20, 2016 at 6:14 pm

    Thank you…don’t know why this happened but your instructions helped me fix it. Running Windows 10 and office pro 2007

  13. Oz said on July 22, 2016 at 3:20 pm

    Great tip! Thanks!

  14. Tracy said on September 1, 2016 at 4:48 pm

    Worked for me, too – thank you!!!

  15. shawn said on September 9, 2016 at 10:25 am

    It’s Worked for me, too
    thank you very much!

  16. Jari said on October 31, 2016 at 11:53 am

    I had a similar issue with Outlook 2013 on Windows 10 and this helped me to fix it. Thank you very much!

  17. Michel H said on November 30, 2016 at 11:08 pm

    Thank you so much. Solved!
    Considering you published this in 2012, incredible not been debugged by Microsoft.
    Thank you again. M

  18. Ziad Bitar said on January 9, 2017 at 2:00 am

    This problem was faced by only one user logging to TS 2008 r2 using outlook 2010.The issue was resolved.


  19. Anonymous said on February 15, 2017 at 5:24 pm

    Great tip. Thank you!!!! If it helps, I had to use the Control Key and the arrow keys at the same time to bring my window back into view. Worked like a charm.

  20. Rochelle said on March 6, 2017 at 11:59 am

    Thank you, this worked !!!!

  21. anom1234 said on May 20, 2018 at 11:20 pm

    Man, you are a fucking god. Thanks a lot, what an annoying bug!!

  22. JC said on October 12, 2020 at 2:14 pm

    Awesome, this post solved the issue. Many thanks!

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