(tested using only "compact_headers-5.1-tb.xpi" )
The manifest.json file should be located inside the extension_name.XPI file located under your Thunderbird profile folder.
eg. in the case of the compact headers extension, it is here:
\profile\extension\compact_headers-5.1-tb.xpi
(& just for ref: the userChrome.css file is at: )
\profile\Chrome\userChrome.css
The "extension_name.XPI" file is just a normal zip file which can be unpacked by any unzipper like 7zip and packed back using the "zip" archive format.
eg: As a test for trying out the extension in a vanilla Tbird, I unpacked the "compact_headers-5.1-tb.xpi" file using 7zip, and modified this one line in the manifest.json file.
Code:
from: "strict_max_version": "117.*"
to: "strict_max_version": "150.*"
After modification of that json file, and repacking all the files back into "compact_headers-5.1-tb.xpi", this particular extension could be installed on Tbird 118.
The repacked .xpi file could also be used to install over an already installed older extension version (also, the zip file name did not even seem to matter, as long as all constituent files were present and zipped in the zip format).
I have only tested this one extension, so would stand corrected if this process did not apply to other extensions as well.
