Add some `padding-inline-end` to `menuitem:not([acceltext])`

I would suggest to add something like the following
```css
menuitem:not([acceltext]) {
padding-inline-end: 16px !important;
}
```
to make the right side of some menus as follows look better on version 139 and above:


Otherwise the right-ends of these menu’s text would be way too close to the right border. The exact value of `padding-inline-end` to add in the above sample, however, was just a quick example of mine to show the effect, not necessarily the best practice.

Start by locating the CustomCSSforFx stylesheet rules that target Firefox menuitem elements, then inspect the handling of menuitem:not([acceltext]) in Firefox 139 and above. Compare affected menus with the supplied screenshot and adjust the end padding so text is not too close to the right border; the exact padding value remains to be determined.

Written by the indexing model from the issue text.