Firefox 3.5 has been launched with significant improvements in speed and new features. However, a small problem when using Firefox 3.5 is plugin not compatible. With small tips below, you will using any plug-in of any version before so easy
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! 
bạn mở cửa sổ Firefox, gõ about:config trên thanh địa chỉ và nhấn Enter. Một thông điệp hiện ra, bạn click vào nút “I’ll be careful, I promise!” để tiếp tục.