screenshot tool has inverted right/left drag if firefox is right-to-left

[emceeaich]

Original issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1633840

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:75.0) Gecko/20100101 Firefox/75.0

Steps to reproduce:

~~Attached~~ Linked video shows this in action.

Set Firefox to have its interface in a right-to-left language. In my case Arabic.
Take a screen shot using the built-in screenshot tool.
Drag the right border to resize the screenshot area.

This was on Firefox 75, on macOS Catalina 10.15.3. Both Firefox and macOS are set to have Arabic UI.

Actual results:

The left border moved.

Expected results:

The right border should have moved.