User Agent: Mozilla/5.0 (X11; Fedora; Linux x86_64; rv:72.0) Gecko/20100101 Firefox/72.0
Steps to reproduce:
Example with duplicating a bookmark (*): open the Bookmarks sidebar (Ctrl-B), select a bookmark there, copy and paste it (Ctrl-C, Ctrl-V), undo (Ctrl-Z) -- the duplicate is removed. Now press Ctrl-Shift-Z.
The developer tools debugger opens.
- The kind of operation on bookmarks (copy, delete, move, etc.) is irrelevant. Pressing Ctrl-Shift-Z after undoing the operation always brings up the debugger.
The duplicate bookmark should have been brought back (undoing the undo). (That's how it worked in previous versions; I can't tell exactly which, but I believe at least until v. 70.)