Hi! Oh, I didn't expect it to be possible, but this work flawlessly! Thanks a lot!! :D Edited the tag from "not possible" to "solved"!

As for the blue circled space, it is removed with this simple line of code:

#vertical-spacer {
  display: none !important;
}

Thanks for the follow-up! I realised after making the post that the spacer is just a simple display: none to apply after getting the tag with the toolbox, so that I managed to do.

As for the rest, it's unfortunate, but I'll tag the post as "not possible". Thanks for letting me know and for trying :D

Sadly it doesn’t seem to work in my case (?)

Thanks, it worked! I had no idea of the change

Oh, you're right on the border image, thanks!

As for the other separator, I totally thought it was native, but after reading your comment I started searching in my css files and I found what was causing the green circled issue. Apparently it's this part of the code here:

/* Add somewhat hacky separator to zoom controls so it looks consistent */
#appMenu-protonMainView > .panel-subview-body::after,
#appMenu-mainView > .panel-subview-body::after {
  content: "";
  display: flex;
  border-bottom: 1px solid var(--panel-separator-color);
  margin: var(--panel-separator-margin);
}

which is a part of one of your styles . I'm pretty sure I had this for well over a year since I basically considered it native. When I removed it while keeping the rest, it seems like it's working just fine anyway. Thanks for the help!

Unrelated side question that I don't think it's worth enough to open a thread on different spaces: to test the code I used both Windows 11 and MacOS Tahoe, I noticed that the quit option is missing from the macOS arrowPanel compared to the Windows one. Here's the comparison (mac on the left, w11 on the right) I probably think this is normal as I noticed this in a native, non modded profile (in macOS the about firefox in "Help" section of the arrowPanel is missing as well compared to the Windows one so it wouldn't be strange if quit one is missing as well) but do you happen to know if what I just said is actually right? It's not really important, more like a curiosity lol

Oh, you're right on the border image, thanks!

As for the other separator, I totally thought it was native, but after reading your commented I started searching in my css files and I found what was causing the green circled issue. Apparently it's this part of the code here:

/* Add somewhat hacky separator to zoom controls so it looks consistent */
#appMenu-protonMainView > .panel-subview-body::after,
#appMenu-mainView > .panel-subview-body::after {
  content: "";
  display: flex;
  border-bottom: 1px solid var(--panel-separator-color);
  margin: var(--panel-separator-margin);
}

which is a part of one of your styles . I'm pretty sure I had this for well over a year since I basically considered it native. When I removed it while keeping the rest, it seems like it's working just fine anyway. Thanks for the help!

Unrelated side question that I don't think it's worth enough to open a thread on different spaces: to test the code I used both Windows 11 and MacOS Tahoe, I noticed that the quit option is missing from the macOS arrowPanel compared to the Windows one. Here's the comparison (mac on the left, w11 on the right) I probably think this is normal as I noticed this in a native, non modded profile (in macOS the about firefox in "Help" section of the arrowPanel is missing as well compared to the Windows one so it wouldn't be strange if quit one is missing as well) but do you happen to know if what I just said is actually right? It's not really important, more like a curiosity lol

Oh, I see. I think that's probably it!

So the only way to have context menus that can be styled with css is disabling widget.macos.native-context-menus, will try this thanks!