I’m afraid I’ve never done something like this before. I wonder how you’d be able to get it…
As a future safeguard, I recommend using something like Dropbox (yes, that was a referral link!). It will keep versions and back stuff up for you! You’ll then be able to access your information from multiple computers – yay!
If it’s cached by Firefox, then you ought to be able to find it in the Cache
folder under the Firefox user profile.
In case you are not savvy to the Firefox profile internals (and in case you
haven’t already tried this), you’ll generally find that folder somewhere
under C:Users[username]AppDataLocalMozillaFirefoxProfiles on Vista,
C:Documents and Settings[username]Local SettingsApplication
DataMozillaFirefoxProfiles on Windows XP, or
/Users/[username]/Caches/Firefox/Profiles on Mac OS X. The files will not
have a .css suffix, but you can order by date and drag the newest files
(skipping the largest ones) into Notepad (or TextEdit) one by one until you
find your CSS.