By chance, I found a "different" version of FF that works great with TW5 and TW Classic, and has no issues with TiddlyFox or any other extension for that matter.
I am using the 32-bit version of Firefox Unbranded, which is compatible with any extension you may want to install, even if the extension is not signed by the Firefox. Moreover, I am using it as portable from a USB.
It is not an official Firefox though, so you may want to use it for developing TiddlyWiki documents/plugins, but maybe not for day-to-day web browsing. More testing required, I think.
This is what I did: Downloaded the latest Unbranded release build FF Windows 32-bit version from:
(have not tried Linux/Mac OS versions yet)
Unzipping the .exe file (extracting it with 7-Zip) produced a setup file and a directory with all the FF files. Changed the name of the directory and copied to a USB.
Started FF from that directory, installed TiddlyFox. That's it.
Comments?
Alfonso