Vimium: Firefox 57: Toolbar panel not shown correctly in overflow menu

Not addressing your issue, but FYI.... Use <c-Enter> to close and save (or Escape to close without saving). You can delete rules by tabbing onto the little "x" and hitting enter.

it might be necessary to do some changes.

This is a bug with Firefox not resizing the pop-up properly, because they've put it in a frame to insulate it from the title bar/back button. This doesn't happen if the bar isn't present (ie. Vimium is in not in the dropdown menu) and happens to all other WebExtensions that are put into the menu.

It's definitely worth going and voting for the FF issue (under details), so that the Proton team give it higher priory.

I don't think we should do anything on our side.

Well yes, I had read that bug already, but if you read all the comments, I'm not sure they will modify the behaviour (the "back button" is a semi-valid point).
So in that case, modifications are necessary (see thebug for examples as well).
Might be better to wait a little bit to see what happens on that bug though. (I did vote, just to make sure :) )

I'm not sure they will modify the behaviour

I'm not sure that they can leave it how it is: most extensions' pop-ups will work incorrectly from their double-chevron menu. There is also no clear (or reliable) workaround. This makes it a bug with that menu, not with the individual extensions.

Naturally, the advice most extensions will give -- and the advice I'm going to give here -- is that that menu is broken, and that you shouldn't use Vimium with it at present.