Before trying this, you can change a setting that saves the sites loaded when you close Firefox (Tools/Options/General/When Firefox opens... show my windows and tabs from last time).
How to run Firefox in safe mode? Close the browser, go to Start, click on Run and type:
firefox -safe-mode
In Linux and Mac, you should open the Terminal and type:
/pathtofirefox/firefox -safe-mode
Another idea is to create a profile that only has the most important extensions. In Firefox, a profile stores your bookmarks, passwords, extensions, web history. To create a new profile, close Firefox and open the profile manager:
* In Windows, go to Start/Run and type
firefox -ProfileManager
* In Linux or Mac, type in the Terminal:
/pathtofirefox/firefox -ProfileManager
Create a new profile, select it and click on "Start". Now you have a blank profile you could use to store the most important extensions. If you don't want to download everything again, there's always the option of duplicating the existing profile and deleting stuff from the clone.
To open Firefox using a certain profile, load the profile manager. It might be useful to have a second (or third) profile if you need to use your computer for a presentation or at work.
Also see:
Reduce memory usage in Firefox
Misc Firefox tips
