The first (and possibly the only one) trouble you run into when you install a brand-new version of Firefox, as for the latest release 3.5, is that most of the used add-ons won’t immediately result compatible with the upgraded browser. The only thing you can “officially” do is wait for some update and face a sort of temporary black-out.

However, in case you can’t really survive without some of your preferred extensions, you can still try to force the compatibility by disabling the corresponding check within the configuration file of the browser. Of course do not forget to backup your data and settings, bookmarks first, just to be prepared against unexpected inconveniences. Therefore, when you feel ready you can go ahead with the following steps and verify that everything still works as expected.

How to disable add-ons compatibility check:

1. Navigate using the address bar to: about:config
2. Create a new boolean attribute: extensions.checkCompatibility
3. Set this new attribute to false
4. Restart the browser

Despite all the precautions mentioned above, I haven’t got to know about any unwanted side-effect. Usually several add-ons are already able to work properly on the new browser whereas in the worst case they just remain disabled.