The minus icon of the zoom controls button in the customization mode is much larger than the plus icon. I attached a screenshot that shows the issue.

Firefox 89 Beta is not affected so it seems to be regressed by a very recent change in Nightly.

Can we track down what regressed this? If it's proton work I guess there's a chance it was uplifted to 89 since then.

If it's proton work I guess there's a chance it was uplifted to 89 since then.

Yes, Firefox 89 is affected now as well.

Mozregression pointed me between 2021-04-29 and 2021-04-30 with the following pushlog: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=f983d8b45e728c19afb32e70c0421a69e8c7a3c4&tochange=18beaf4ff7f32381d875efb395fe1daec664a7dd

And Bug 1693856.

The "+" icon in the customize palette ends up squished due to this rule added to center the icon while in the toolbar:
https://searchfox.org/mozilla-central/source/browser/themes/shared/toolbarbutton-icons.inc.css#352-355 - which effectively reduces the icon width to 9px in this context.

Some background: the proton "+" icon is aligned to a 16x16 grid and as the vertical stroke is 1.25px wide, it cant be completely centered. The icon designer chose to put the vertical stroke (for this and all the proton icons like it) on the 8th pixel/column. In this context, it looks a little better pulled 1px to the left, so the new rule was to apply 7px left/inline-start padding rather than the normal computed 8px.

I think margin-inline-start: -1px might work better and avoid this fallout in the customize palette.

We're unable to reproduce this, it seems to have been fixed.

Confirmed. Fixed by bug 1760610 according to mozregression.