[unexistance]

I can relate to this, tho for me it was TiddlyWiki(-legacy), and UnMHT

you can use the ESR[0] release with the extension AND disabling auto-update, and use -no-remote paramater to run multiple instance FF...

So in your case, you can have two instance, one FF running ScrapBook, the other is new-and-shiny FF... I used this setup daily with multiple FF running at my whim, didn't feel clunky

p/s: ScrapBook sounds awesome, I myself did saves PDF version of website, where it still can be indexed/searched properly

[0] https://ftp.mozilla.org/pub/firefox/releases/