The fast and best way:

Get the password and username backup utility free (google it) and backup data.
go bookmarks -> organise bookmarks -> import and backup -> backup

uninstall firefox (keep personal data)
install new and updated firefox (some versions let you import it on install, if so you shouldn't need the backups)

everything is where it was like it was and firefox 100%faster and 100% more stable.

hope I helped.