>>ReadCa+(OP)
I switched to LibreWolf (A fork of Firefox with privacy enhancements and ublock preinstalled) and haven't had to deal with anything like this since.
>>mattl+S9
I'm guessing that was because Debian refused/wasn't able to sue the Firefox branding and they extended that to the user agent. So a ton of sites just saw Iceweasel as the UA and broke because it was unexpected.
>>mattl+qc
LibreWolf isn't strictly a fork of Firefox, it's a preconfigured installation that locks things down and disables anti-features. So it's strictly accurate to use the Firefox user agent, because that's what actually gets compiled.