"Forget" tool.
When I tried the "Forget" tool, it wiped out all my tabs being managed by the Simple Tab Groups (STG) extension. I had to restore the last backup. I won't use "Forget" again!
Wšykne wótegrona (4)
Note that "Forget" is a 'panic' button that comes with a warning and should be used with most care.
It is better to use "Clear Recent History".
I'm a little puzzled why Forget would affect add-on data. Clearing your browsing history (with either the Forget button or with Clear Recent History -- with Browsing & Download History selected or by setting Firefox to clear history when it closes) will erase Firefox's own session history files. Maybe Simple Tab Groups relies on Firefox's files, and doesn't store a separate list? I don't really know how it works, but you could ask its author how to accomplish what you were trying to accomplish without losing so much data.
Forget closes all open windows and opens a new window and that might cause STG cause losing its data.
STG does keep backups of the open tabs by group, so I was able to select the latest backup and restore all the tab groups. Since "Forget" apparently closes all your tabs and starts a new window (in addition to deleting your browsing history back to the selected time frame), STG is left with no open tabs. Bottom line is that I won't be using Forget. I took the icon off my toolbar.