Firefox's capabilities seem ridiculously limited to me. There are valid reasons to sometimes use Firefox (or other browsers that are based upon it), but Firefox is in general a pitiful experience compared to most other browsers on the Mac.
You might consider Brave, which is another browser based on Chromium.
It is already pretty mature (and not senile like Firefox). Like Safari, Orion uses WebKit, and in use it feels a lot like Safari, except that it: is a lot snappier; uses (with varying degrees of success) Chrome and Firefox plugins instead of Apple's system; and it has a different set of limitations. 
Those limitations are generally things that have not been implemented yet rather than things that have been left with rough edges (
). Support for Orion in KM now looks more feasible, and in the meantime, there are workarounds.
As I'm sure you know, there are plenty of other alternatives too.