Rated 5 out of 5 stars

This is a fantastic add-on. Let's congratulate the author and not attack so hard for the occasional glitch. I also cannot use TB at all without TFSC. I've also had the TFSC button disappear, and it turns out this is because of an easily removable date check in the code. The check seems to be trying to ensure that TFSC has been updated within the past 90 days. So if the author goes longer than 90 days before making an update, the button disappears. When this happens, you can fix it yourself by making a trivial modification to the official release - this is preferable to downloading and installing software from an unknown source.

It seems like a recent update has fixed the problem, but it could happen again any time that an update is not issued in 90 days. I took a look at the code in the new update, and the 90 day check is still there.

To fix this yourself without downloading untrusted software:
- download the "xpi" file for the latest TFSC release.
- change the file suffix from xpi to zip
- unpack the zip file into a work directory
- in your work directory there will be a file called "bootstrap.js". In that file, search for a function called "compile:function()". Comment out the line that begins with "if(new Date().getTime()".
- Save your changed bootstrap.js
- zip the contents of your work directory into a new .zip file. Note that you need to zip the contents of the work directory, not the directory itself.
- change the suffix of your new zip file to xpi.
- install your xpi file as a thunderbird add-on.
- set automatic updates to off for your new version

Thanks to the original comment by happyone that had a pointer to the place to fix.