Latest version is great on Firefox, has problems with SeaMonkey Rated 4 out of 5 stars
Works great in Firefox. No issues, and I can download youtube videos for offline viewing to my heart's content.
The latest version (1.6.4) doesn't seem to be compatible with SeaMonkey as it prevents links from loading and new tabs from opening. I noticed that there are no SeaMonkey versions listed for 1.6.3 and 1.6.4, but for some reason several computers auto-updated from 1.5.18 to 1.6.4 anyway. A manual downgrade to 1.5.18 fixed the problems.
Thanks for your bug report. Version 1.6.5 should fix this: https://addons.mozilla.org/en-US/firefox/addon/download-youtube/versions/