Does anyone got an idea ?

IMO, that Malwarebytes Browser Guard is 'faulty' or could have been coded better. Something which, IMHO, should have been fixed before releasing that extension.

The >> for the overflow is pinned to the right-side of the Navigation Toolbar (at least my attempts to 'shove it' to the left hasn't worked when I tried that for you).

Beyond that all I can think of doing is to use Customize and move that particular extension button to the Navigation Toolbar; and maybe stop using that Overflow feature, especially if you have any other extensions there that have wider than 'normal' drop-down displays. That width can be compensated for within the extension base code (I think), but hard to do with "user hacks" via userChrome.css .

Perhaps another support contributor has other ideas. I have gotten 'rusty' in the UI / Toolbar mods area over the last 4 or 5 years; tired of fixing garbage like that every few years when it breaks, again or was made poorly to begin with.