Great concept, the highlighting/capture works well, the full page view of snips is great and the highlight popup doesn't overlap Textmarker popup :), but I won't be paying $3.99/month to save images, write notes and export.
$3.99 1-off I would pay, and probably a lot of others would too.
Without export it has very limited use.
Why do you want my email address and an account? (It's persistent when you select use as guest, it should be saved locally).
Why isn't there an opt out for spying on usage. I'm unlikely to use anything which spies on me.
PDFs open in an addon tab which is a clever workaround (you could add a prefix "snip" to the page title). Highlights aren't saved when the PDF is saved to file, and the highlighted PDF tab is lost when FF is closed (the snips are kept in the my snippets page). Neither is much of a problem so long as you are aware of them.
Is there a way to remove the highlight? It's not in the right-click context menu or the pop-up, and deleting the snip and refreshing the page doesn't remove the highlight. Should be able to remove the highlight from the page, independently of the snip.