EDIT: I've read your edit just now, because I didn't see it (the one line at the bottom)... Sorry for late response.
Go to Firefox Options - Advanced at the bottom - Network
Try to Clear Cached Web content and Offline Web content.
If it does not help you will have to wait for update. Sometimes cached files are still in use, so it's kinda hard to clean up everything. In next update I will provide emergency button in Tree Tabs options and load backup buttons in toolbar. In your case you probably will not be able to recover last session, but in the future when that happens again, yes. I will provide 3 backups. I'm thinking to provide save backup to hdd, but I'm not sure if I will be able to finish it for this update.


My brother had this same problem yesterday, as few other people. If options page shows up without a reason it means localstorage data got corrupted or some part of it cannot be read. Same goes for your trees. Some have just a blank white page instead of the sidebar. Looks like Firefox has some problems with retaining localstorage, it might be that some process close unexpectedly corrupting data while saving it to hdd. In my experience it happens from time to time, but it's more frequent with last few updates of Firefox.
They may be in hurry to release 57. I don't know just guessing.

You have to clean local storage to fix it, otherwise problem can come back.
https://stackoverflow.com/questions/18877643/error-in-local-storage-ns-error-file-corrupted-firefox

In a new version of my extension I will save a lot less data, but I cannot guarantee all will be perfect and I have a lot to fix to release it. Anyway it is a good idea to save your theme like you said.