We would like to advise that Sothink Web Video Downloader for Firefox V5.4 is compatible with Firefox 3.5 indeed.
Please uninstall Sothink Web Video Downloader for Firefox completely from your Firefox, and then reinstall it for a try.
Moreover, please clear the cache in your Firexfox before you reinstall the program.
Please go to the following page to download the latest version of Sothink Web Video Downloader for Firefox:
How to Install Web Video Downloader for Firefox? Please check it here:
If you have any question, please feel free to contact us at email@example.com
Thanks for your support!
Hope you enjoy Sothink products and have a nice day!