button dissapears - can't test Rated 1 out of 5 stars
button dissapears - can't test - incompatible with some other plugins
ed.: button dissapears every time some addon is installed/remove - that's unaceptable - had to remove
Please let me know your Firefox version and add-on list by e-mail (email@example.com), so I may debug this problem and may provide a patch.
UPDATE: I have seen this behaviour being caused by certain SDK-based addons, such as an old version of MediaPlus. Updating Firefox to the latest version might solve this issue.