New tab icon looks strange

[RealSkeptic]

When Tabkit is active, the new tab icon changes into something a bit strange.

On my 4k display, it looks like a small dash:

On my 5k display, it looks like a flattened plus:

I don't seem to have any preference where I can change or choose the new tab icon.

The machine is an iMac retina 5k, with a secondary 4k monitor, OS X El-Capitan.

[PikachuEXE]

What's the FF version & Tab Kit version?
I tried horizontal mode with FF 45 there is no such issue
Also are you using default theme?

[RealSkeptic]

Note that I'm on a Mac, your screenshot seems to be from Windows or Linux.

Firefox 46.0.1
Tabkit 0.12.14

I'm using a non-default theme, but note that if I disable tabkit and restart Firefox, I get a proper "+".

Oh, and I just discovered an important thing: I use Tabkit in multiple-row mode. If I disable that, I get a better-looking "+", though it still has a strange aspect ratio on the retina display:

Retina (without multiple rows):

4k display (without multiple rows):

[PikachuEXE]

This is what I have with FF 45.1.1 & TabKit 0.12.14 (OSX 10.10.5)
Which seems fine to me
Maybe you can try 45.x to see what happen

Although I found another issue (unrelated):

[RealSkeptic]

You have the "new tab" button separate from the tabs themselves. I'm not sure if that's the default for FF or not, but I have it always after the last tab. You can change it by going into the Firefox menu and clicking "customize", and then drag it to the tab area. When you "exit customize", it should appear at the end of the tabs list.

This is Mac-specific for sure, as now I'm writing from a Linux machine and it handles the "+" correctly, with or without "multiple rows", with or without the button at the end of the tab list.

[PikachuEXE]

I have tried to drag the new tab button into tab area but almost it will go to the left:

I am using default theme BUT also using Classic Theme Restorer
Maybe you can show me your settings for it so I can try again?