hello, you won't lose any extensions unless one of them is explicitly specifying it will only work with firefox 21 & above - which is rather unlikely (in this case it would show up as disabled in the addons manager).

You can also give the Firefox 22 beta version a try to see if that works better.
It is usually not a good idea to downgrade.

• http://www.mozilla.org/en-US/firefox/all-beta.html
• http://www.mozilla.org/en-US/firefox/beta/

Be sure to do a custom install and install this version in a separate program folder and create a new profile. I.e. do not launch the beta version after the installation has finished, but use the Profile Manager to create a new profile and use a desktop shortcut to start this version with this profile (-P "Profile Beta").

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox
• http://kb.mozillazine.org/Bypassing_the_Profile_Manager

This has been reported to the Firebug developers: Issue 6458 - fbug - Firebug 1.11.3 makes webs incredibly slow on Firefox 21. I do not see any sign of a resolution there, although some have reported luck in upgrading to a different version.

In another thread, a user here reported that resetting Firebug settings resolved the issue (see attached screen shot). Another user said that helped, but after a few hours, the problem returned. It sounds as though we don't fully understand the problem yet...

https://support.mozilla.org/questions/960066#answer-439451

Edit: Upon further review, it appears that the reset might turn off the Console and Script panels, which are the panels that seem to cause the slowness. That definitely is not a long term solution.