That's a nice idea... unfortunately for now it's not possible to do that, at least not to a text file placed in an arbitrary file system location. The new WebExtension API only gives very limited ways to write data to the user's computer. If your downloads are automatically saved to a given directory, then it's possible (and you would get all that saved tabs in your Downloads folder). But for those who have a window prompting to save the file, it would display it every time the data is going to be saved... A possibility would also be to store to some "local storage" the API allows the extensions to use, but as far as I know, it is not possible to write arbitrary files there (e.g. no text files). I'll possibly make something about this in a next release, where I mainly want to add the possibility of reloading the files and opening the saved tabs again.