On Linux, you can go to your Terminal and run:
/path/to/firefox/firefox -safe-mode
What works for me is usr/bin/firefox -safe-mode
Uninstalling and reinstalling Firefox is rarely the fix for any problems Firefox might be having.
Make sure you have the latest versions of Flash and Java.
You can check the plugins you have installed by typing about
:plugins in the address/location bar.
If the problem is with Firefox then try running
Firefox in Safe Mode
If all is well, you have a "bad" theme or extension that needs updating, replacement or removal.
Tools Menu -> Extensions Dialog, each extension can be individually disabled using its right-click/context menu.
Disable individual extensions until you find the one that causes problems. Uninstall it.
Tool>Themes>uninstall follow the same steps as a bad extension
If the problem still exists it could be a bad profile in which case you need to
Create a new profile.
You can "restore" most of the information from your old profile to your new one.
Restore your profile
These are the important files you might want to save:
# bookmarks.html Bookmarks.
# prefs.js Current user preferences.
# user.js User-added preferences (overrides prefs.js every startup).
# cookies.txt Cookies.
# hostperm.1 Cookies permissions, image permissions, and extension website install permissions list.
# signons.txt Saved passwords. Requires the "key3.db" file to work.
# key3.db Supplements password file.
# userChrome.css, userContent.css