I have narrowed down the problem a bit but still have no solution
The 403 Forbidden error
only occurs for me when I'm using Pale Moon browser to access a MyBB forum that is installed on X10's Server xo7.
All 3 of those things have to be true.
If I use another server then everything is fine
If I use another browser (Internet Explorer) then everything is fine
If I use another forum (SMF) then everything is fine
There is something about the Pale Moon + MyBB + Server xo7 combination that just doesn't work.
That combo worked for me for a long time using various versions of Pale Moon and MyBB, but then a few weeks ago something on Server xo7 changed and the 403 error pretty much shut down my website.
I did a little searching in the Pale Moon Forum and found a post about a similar problem but their solution did not work for me.
See:
https://forum.palemoon.org/viewtopic.php?t=18430