User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:90.0) Gecko/20100101 Firefox/90.0

Steps to reproduce:

1. Put the "Library" button on the toolbar
2. Go to about:newtab
3. Click the Library button
4. Click "Bookmarks" or "History"
5. Click the back arrow

Actual results:

After clicking the back arrow and returning to the top level menu, none of the other options are clickable. Additionally the rest of the browser seems to be non-responsive. This does not seem to happen in normal tabs.

Dragging the window around my desktop seems to unfreeze the browser.

Expected results:

Clicking back into the top level menu should not freeze the browser, and the other options should be selectable.

It actually does happen in other tabs that are not about:newtab.

Unfortunately I cannot reproduce myself, though 1705085 looks related. Can you collect a profile of the hang with the Firefox profiler? https://profiler.firefox.com/

This is almost certainly bug 1701451.