I've created a firefox userChrome.css theme...

[GorrillaRibs]

I've made (modified) a couple of themes, both based on kurogetsusai's firefox-gnome-theme, to fit better with Tetra-dark than the regular firefox theme - one with tabs on top, and one with tabs on the bottom. The majority of the code is from the original, with colors and other minor tweaks to fit with Tetra. I'm still working a bit on them, but they're both in a pretty solid state at the moment, so I thought I'd attach them here (if a pull request is more appropriate let me know and I'll open one - I thought this would be easier to start with) if you'd like to include it in your repo in some way. The original code was released with unlicense, so I'm pretty certain there's no issue with including it here under the GPL. Thanks for the great theme!

chrome_bottomtabs.zip
chrome_toptabs.zip

[hrdwrrsk]

Untested. I use chromium as main browser, but I will try since firefox isn't installed because i'm switching again to Arch Linux (from Debian testing/unstable)

[GorrillaRibs]

Sounds good - if it affects anything I noticed some issues with the tabs on top version after I uploaded it (mostly just container tab lines not displaying), so here's the updated version:
chrome_toptabs.zip

This version also works properly with firefox's CSD

[hrdwrrsk]

in firefox dev edition it looks strange af
especially popover item(s)

[GorrillaRibs]

Strange af indeed - I'll work some more on it - the tabs-undereneath looks more complete (that's the one I use day-to-day), but both themes do look strange with CSD (as do the popup buttons). I'll continue working on them. The back/forward buttons look dark because of how it works with some icon themes, the only fix I know of is to edit the close-button-icon-filter in firefox-gnome-theme/ui/gnome-3.26-dark.css (either invert(100%) or none), it's different for almost every icon theme I've tried so far). Current versions of both included here, but I don't think they solve any of the above issues yet:
(also pictures of the bottom tab version for reference)
chrome_bottomtabs.zip
chrome_toptabs.zip

[GorrillaRibs]

Well it's been a while (got caught up with life), but here's a much-improved version of the theme in line with the updated firefox-gnome-theme (with tabs on the bottom right now, I'll work up a version with regular firefox tabs either tonight or tomorrow). CSD also works properly now, which is nice.
chrome.zip

[GorrillaRibs]

Took me a little while longer than I thought to work out a couple bugs, but this should be done - tabs on top like normal, CSD works, etc. Only issue is due to a firefox bug, the titlebuttons set by the theme are always scaled at 1x rather than 2x, even on high dpi screens, so I don't really have a workaround for that. Otherwise it should be fine though.
chrome.zip

[rdrms]

I've also spent a little time messing around with firefox-gnome-theme, but with more emphasis on the light theme which I personally use. You can see the results here: https://github.com/ryanrms/firefox-gnome-theme

Just trying to make it match Adementary a little bit more closely, any help would be appreciated since I'm a humble amateur just copy and pasting stuff. But it's definitely an improvement on the main firefox-gnome-theme in terms of matching the Adementary style.

Thanks again for creating such a great subtle theme for gnome!