When you start librewolf or whatever you switch to (librewolf is what I use) it will have everything from Firefox including whatever your last session was (if you save your session) and all of your history and bookmarks and even addons and UI customization etc.
Switching your firefox profile to a firefox fork is super easy peasy squeezy cheesy peas.
You just need to follow the steps for moving your profile in this Firefox support article: https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
When you start librewolf or whatever you switch to (librewolf is what I use) it will have everything from Firefox including whatever your last session was (if you save your session) and all of your history and bookmarks and even addons and UI customization etc.