Where Firefox for Android is keeping add-on generated files? I want to find a folder, where Firefox for Android is keeping files generated by installed add-ons (list of saved tabs in OneTab, for example).
I've tried to do a full root file system search (e.g. onetab.html, store.json) and looked insede specific app folders (e.g. /data/data/org.mozilla.firefox). Found nothing but the .xpi extension files.

According to Help @ one-tab.com, tabs data is located in C:\Users\Username\AppData\Roaming\Mozilla\Firefox\Profiles\Profile Folder\jetpack\extension@one-tab.com\simple-storage\store.json for a Windows desktop version. On Android, using rooted Amaze, I've found nothing similar.

Where should I search next? Are those files even visible for a root user?
Browse into the location /data/data/org.mozilla.firefox/files/mozilla/. Among many directories there would be one directory whose name would end with .default. That is your Firefox profile directory. Within that directory, you can find what you are looking for. Extensions (xpi) are saved inside the extensions directory. Some extensions may have data inside the directory browsers-extension-data while others may have it stored inside storage/default.

You can access those files by entering the URL file:///data/data/org.mozilla.firefox/files/mozilla/ in your Firefox browser. If you want to access them using a non-Firefox app, such as a file explorer app, than you need to have root access.