Running SP 5 Pro in Windows 7.
Upgraded to Firefox 18. Firefox 18 runs well but no SP auto-fill plug-in available from Mozilla.
Went to SP site and found update to 22.214.171.1248, which is supposedly compatible with Firefox 18 and includes auto-fill plug-in.
After 126.96.36.1998 installed, Firefox 18 repeatedly crashes.
When auto-fill plug-in is disabled, Firefox 18 again runs without problems.
Conclusion: auto-fill plug-in not properly updated for Firefox 18. It's all buggered up.
Don't know if this helps:
Without auto-fill plug-in, Windows Task Manager reports firefox process as using about 186K of memory.
With auto-fill plug-in, Windows Task Manager reports firefox process as using about 3300K of memory.