Firefox since yesterday was checking for add-on compatibility on every browser start. It displayed the window that you only see after upgrading to a newer or downgrading to an older version of the browser.
I first though that this was caused by the excellent sandboxing software Sandboxie, but this turned out to be not the case. I tested it by disabling Sandboxie before running the browser. The add-on update check was still displayed every time I started the browser.
I then checked if it was an issue with the prefs.js or another file that does not get updated properly forcing the browser to repeat the same process on every start.
While I was looking through the Firefox profile folder I noticed a user.js file in there. And that file, according to Mozilla does not exist by default. The Firefox developers note that that "once an entry for a preference setting exists in the user.js file, any change you make to that setting in the options and preference dialogs or via about:config will be lost when you restart your Mozilla application because the user.js entry will override it".
This for my situation meant that the user.js file was blocking Firefox from updating properly. I have no idea how and why it was created in the first place, but decided to create a backup and delete it to see if it resolves the issue.
I closed Firefox first, copied user.js from the profile folder to the Windows desktop, and deleted the file afterwards. Firefox on next start did not check for add-on compatibilities anymore and the browser seems to have returned to its former self.
It is also interesting to note that the user.js file has not been created again by the browser.
Hope this helps Firefox users who are encountering the same issue that I did. I suggest you create backups of the file just in case for an option to restore it if this does not resolve the issue you are experiencing.
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Ghacks is a technology news blog that was founded in 2005 by Martin Brinkmann. It has since then become one of the most popular tech news sites on the Internet with five authors and regular contributions from freelance writers.