The basic idea was to inform users and developers alike about the top add-ons that slow down the start-up of the
Firefox web browser. Slow Performance data has been added to the Mozilla Firefox Add-on Gallery.

The biggest offenders back then slowed down the browser by 74%. A Firefox user who’d install the top 5 add-ons of the list would slow down the start of Firefox by almost 300%.

The situation looks as grim one month later. The top five add-ons now slow down the browser by more than 350%. Some add-ons that have been previously on the list are not found in the top 10 anymore, but most add-ons that have been in the top 10 back then are still there, many with slower start-up times than before.

Mozilla has modified the top listing slightly. Only a top 9 list is displayed on the Slow Performing Add-ons page on Mozilla. Back then it was possible to load a top 50 list, which does not appear to be possible anymore.

Operating system filters have been added to the page which have not been there before, making it now possible to display the slowest add-ons for Fedora, Mac OS X, Windows 7 or Windows XP instead of a mixed listing.

If you look at those listings you will notice that only the add-ons of the top 9 listing are displayed. The start-up performance on the other hand differs widely. The worst offenders of all lists are SimilarWeb and Personas Plus under Windows 7. They slow down the startup time of Firefox by 223% and 202%. Compare that to the 25% or less add-on start-up time on the tested other operating systems. These numbers may be flawed, and it is very likely that they are considering the huge difference between Windows 7 and all other tested operating systems. While a flawed test is the most likely explanation, it could on the other hand be an extension specific problem instead. Only re-tests will tell.

It seems that my assumption was wrong. Firefox saves the last 50 visited websites of a single session in memory which means that it could add up quickly if you visit lots of content filled websites. The reason behind this is that it is faster to access a already visited website if it still resides in memory instead of loading it from disk cache or from the server.

50 sites on the other hand seems a lot to me. I usually do not visit sites again that I visited a while ago – I tend to visit many different sites and only a few ones more than once daily. The fix is relatively simple. Type about:config in the Firefox address bar and search for the term:

browser.sessionhistory.max_entries

Right-click the entry, select modify from the list and change it to a lower value. I set it to 5 which works well for my surfing habits. Other values might be better for yours.

source: A2Z