I’m guessing that stands for beta?
Probably best to wait for a final release then. 
A major release later (149.0b5) the issue still persists so I’m guessing unrelated to the “beta” status
Can someone not running the beta channel (regardless of version) confirm that behaviour shown >here<?
I just wanna eliminate that variable. And I don’t have a firefox in use anymore.
EDIT
HOLD ON … It’s when you are on the unread page, I didn’t realize that before.
Yes, it does happen for me as well. Can confirm now.
Oh, you have no idea! 
I did install Firefox on my gaming system (Ubuntu 25.04) to test and I fail to replicate your result. Maybe it’s an arch package thing? My Waterfox looks normal as well on Ubuntu, fedora and macOS.
Yes, it also happens when I’m not on the unread page and just hover over the button.
Yeah. That looks like a CSS thing. Easiest way to fix is probably to switch the color of the box or to disble the color change on hover and selection so that the text stays white.