I fixed the trailing space bug for me (V2.95).
1. Disable (stage) the addon or quit FF. Must be done, otherwise FF blocks the xpi file against updates from 7zip.
2. Open the xpi with 7zip, edit ep.js
3. In line 20, remove the ' +" " ' at the end of the line.
4. Save, update change into xpi
5. Now enable the addon again, if it was disabled.

Now the option "Capitalize Every Word" does no more add a trailing space. Not extensively tested for other side effects, but looks like it's working until now.

Doesn't work if the text spans multiple lines. Also adds a trailing space for some reason. Otherwise does the job.
Using FF 70.0.1