RecallMonkey as of v13 has been rewritten in the Addon SDK. There's some important changes to note:
There is no longer a menu item, but there is an icon in the addon bar, that feature is not built into the SDK, I might add it later, but it seems like they're discouraged because of the Firefox menu on Windows in the new UI.
The shortcut has changed to Ctrl/Cmd + Shift + M. The addon SDK wasn't playing nicely with the original shortcut.
Sorry about those and if they cause any confusion.
On the bright side, the tab is now persistent and can be pinned and you won't lose its search capabilities on refresh. This needed a major change on the backend. The code is under development here and I'll move it to the prospector repo as soon it is somewhat stable.