If TubeBuddy does not show up when you go to youtube.com in FireFox, there are a few possible causes.
Cause #1) It is possible that TubeBuddy isn't installed on FireFox. To double-check if TubeBuddy is installed, please do the following:
1) Click on the main menu in the upper right corner of FireFox, and choose Add Ons.
2) Verify you see TubeBuddy installed.
3) If you don't see TubeBuddy listed, go to https://www.tubebuddy.com and click on the link to install TubeBuddy. If you do see TubeBuddy listed, move on to Cause #2.
Cause #2) If you are not currently logged into your YouTube account, you won't see the TubeBuddy menu on youtube.com. So verify you are logged into a YouTube account when on that site. If that isn't the issue, move on to Cause #3.
Cause #3) FireFox supports profiles so that multiple people can share a computer but each maintain their own bookmarks, history, etc. Sometimes these profiles can become corrupt in a way that prevents TubeBuddy from showing up.To see if this is the issue, follow the steps below.
i) Open FireFox, enter about:profiles in the URL bar, then hit enter.
ii) Click the 'Create a new Profile' button. You can take the defaults or customize the new profile.
iii) Under the new profile, click the button 'Set As Default Profile'. This will make it so that when you open FireFox it uses the new profile.
iv) Close all open FireFox windows, then open the browser again.
v) Go to https://www.tubebuddy.com and click the link to install TubeBuddy again. You'll need to do this since you're using a new profile.
vi) Go to youtube.com and sign in to your YouTube account. At this point, you should see the TubeBuddy menu and icons on screen.
If using a new profile fixed the issue with TubeBuddy not being displayed, then the fix is almost complete. The last thing you need to do is choose if you want to migrate any data from your old profile (e.g. history, bookmarks, saved data, etc.) to your new profile. If so, there are various articles online describing how to do this, or you can move things manually.
Hopefully that helps, but if you are still stuck please let us know!