Is it just me or does everyone have poor visibility of the current tab?

Vertical tabs, current tab not clearly highlighted/indicated enough, dark mode.

My "theme" is I think the 'default' on Manjaro gnome, dark mode.

I imagine that depends on the theme you're using. In my soft yellow theme, the current tab is very visible.

This has been bothering me since the launch of proton, their current designs paradigm

You can have a custom theme with way more contrast

You can use userChrome.css to change the border color of the active tab. Here's what I use (credit)

.tab-background[selected] {
  border: 1px solid !important;
  border-color: lightgray !important;
}

yep, same here, I like the theme I am using, I am not changing it because of Firefox, instead I changed Firefox theme to dark (from default).

Same, but then switched to Alpenglow