Google is now offering extensions for Firefox: Google Firefox Extensions.
- Google Toolbar - the toolbar that was previously IE only
- Google Send To Phone - send messages to phones, US only
- Google Suggest - adds Google Suggest to Firefox search bar for Google (does not effect other searches)
The last two are Firefox exclusive (last one definitely is, as IE has no search bar).
Edit: Google Blog: The platypus of the Internet (praises XUL technology, which is what Firefox and Firefox Extensions are built on). mozillaZine: Google Releases Toolbar and Extensions for Mozilla Firefox