Hi Lex,
Unfortunately, if Pale Moon can support only KeeFox 1.4.8 you will be forced to stick with using that old version.
Since all versions of KeeFox prior to 1.7.3 required a (more or less) matching version number for KeePassRPC, this also means it is impossible for you to install a newer version of KeePassRPC into KeePass.
In turn, this limits your maximum possible version of KeePass to 1.38 (due to incompatible changes made by KeePass in version 2.39 and higher).
We can’t support any version of KeeFox earlier than 1.7.3, and even then, we would only work on it if a critical security vulnerability were to be found. After the end of 2018, even this support is likely to be stopped unless other people step up to take over the maintenance work at that point.
If Pale Moon enables support for WebExtensions (the latest browser add-on technology) it should automatically become compatible with Kee 2.x but if there are minor tweaks that we need to do to enable a Pale Moon WebExtensions variant to function, we’d definitely consider that. The same goes for any other browsers based off of old versions of Firefox.
If you still come across a reason to get the debug information, I have just added some more detailed instructions to the troubleshooting page: