The hover tooltips aren't supporting square corners

### Is your feature request related to a problem?

The hover tooltips aren't supporting square corners, they are rounded.

### The solution I'd like

The hover tooltips should have support for square corners through new proposed `userChrome.rounding.square_tooltip` `about:config` item.

### Screenshots or video

If you don't know what tooltips I'm thinking, then I will attach a screenshot:

### Alternatives I've considered

_No response_

### Additional context

_No response_

No files or tests are named. Start by searching the repository for existing rounding settings and tooltip styles, then trace how about:config options are exposed. Done means the proposed userChrome.rounding.square_tooltip setting reliably gives hover tooltips square corners without changing the default rounded appearance.

Written by the indexing model from the issue text.