html tag is setting
color: var(--fg)
Thanks for the heads up and I'm sorry about that, I can't seem to recreate it in Firefox at the moment but I'll get to the bottom of it. Are you in automatic/dark/light mode? Your Firefox version would be a big help too if you were happy to share that.
In the meantime, you can see the spec here: https://gitlab.com/NUMTechnology/Domain-Verification/docs/-/...
Essentially, the issue is that Vimium's stylesheet also set the --fg variable. While your website sets it to #000000 or something like that, Vimium sets it to #FFFFFF and for some reason that interferes with the webpage. Alas, one more reason for me to stay with SCSS haha.
Even more interesting is that the same issue does not happen on a Chromium browser with Vimium with the exact same stylesheet, meaning the global stylesheet pollution only happens on Firefox.
Anyway, I would say this is not a problem with your website but rather with my (beautiful) Vimium stylesheet [1]. Still, you could fix this issue by converting these CSS variables to SCSS ones, so they are hard-coded variables on runtime, or perhaps by using !important to make sure it's not overwritten.
The stylesheet I'm using is not the default but rather a fork of a pretty specific one, so I'll also make the changes there to stop using CSS variables.
By the way, since you asked I'm running Firefox 112.0.2 (64-bit), Red Hat fedora - 1.0 build, although it's not related to Firefox at all.
edit: just prefixed all css variables in the stylesheet with an identifier and everything is working now :)