Steps by Steps:
1. In address bar type about:config and press Enter. the new windows will show warning, click to I’ll be careful, I promise!
2. Click right mouse in any where and choose New –> Boolean
3. The new dialog will show, enter extensions.checkCompatibility and click OK, choose value is False
4. The same step 3, right click to create new boolean extensions.checkUpdateSecurity and choose value False
5. Restart your Firefox to finish.
And now, all the plug-in was before you ever use on Firefox 3.0, even Firefox 2.0 can install correctly in Firefox 3.5. And you can still use Firefox 3.5 with all plug-in already installed in previous versions.
Wish you success!