There isn't any post #14 in that thread that you pointed to.
Well, if you can’t find it, I can’t hold your hand.
If you insist on using an outdated version of Firefox, then you must use a version of the extension that is compatible with that version of Firefox.
Is Firefox 115 compatible with Windows 7? Yes.
Is 115 secure? Yes, at least for the time being.
Is 115 up to date? No.
That’s partly what your bank was complaining about – your outdated version of Firefox. (Sadly, the banks seem to incorrectly equate “outdated” with “insecure” . . . but that’s a topic for a different thread.)
If you want a version of Firefox that is both up to date and Windows 7 compatible, use R3dfox. Version 142 is consistent with Firefox 142 but modified to still work on Windows 7.
FWIW, note that Ray’s 0.5.0 extension was compatible with Firefox versions 48 through 131. Clearly, he was loath to cut off older versions of Firefox arbitrarily, so there must have been a good reason. Perhaps there was a change in the Firefox 132 open source base code that required him to recode his extension, with the consequence that the extension update was incompatible with the older Firefox versions.
Note that if you insist on using the outdated 115 version of Firefox, you can still do so with the 0.5.0 extension if you manually tweak the userAgent string. It's very simple to do, takes about 30 seconds, and it will work. Start by selecting one of the later options from the extension’s quick-pick list (e.g, “Firefox 126.0”), then in the “userAgent” line manually change the “126” to “140” or whatever you want, and then click the "Apply" button.
So take your pick. Use Firefox 115, the 0.5.0 extension, and manually tweak the userAgent line. Or use R3dfox 142, the latest (0.6.6) extension, and pick the userAgent from the more up-to-date quick-pick list. It's your choice. You’re making a mountain out of a molehill.