about:tor jumps on tab switch (when bookmark toolbar is visible)

Introduced by ESR 128. Steps to reproduce:

1. Make sure you are using the default "Only show on new tabs" for the bookmarks toolbar.
2. Open "about:tor".
3. In another tab, open "example.org".
4. Switch from the website to "about:tor".

The content of "about:tor" will visually jump up slightly on return.

I suspect this was caused by bugzilla bug 1705215.