easy access/edit/backup and then restore saved data using the integrated Bookmark Manager (even on smartphone / tablet) even if the addon is not installed

Why save sessions as bookmarks? - all the data saved will be there no matter what addon you may use in the feature

They are proof that our openness about our data formats means that you do not have to fear data lock-in.

It’s this third way that we avoid lock-in that is relevant to today’s topic. Our data format design is specified well enough so that people with no connection to AgileBits can write software to be able to handle it.

Want to keep your users? Just make it easy for them to leave.

Users are starting to realize, however, that as they store more and more of their personal data in services that are not physically accessible, they run the risk of losing vast swaths of their online legacy if they don't have a means of removing their data.

Export and migrate your data between hosting options at any time

And since it’s just a client, you can always use the todo.txt format text file created by it in the very beginning with any other client. Give it a try.

Most to-do lists give you no control over your data. Your tasks live inside the app, not in a document you can edit, and syncing is handled by whichever company made the app. If you don't like this, todo.txt is a great alternative.