That's how Instapaper did it back when I used it (may have a full-fledged add-on now, but I don't know for sure).

The only drawback, for me, was the added bulk of the Bookmarks bar. I like to slim it down, so having the single button has been great. (Also, being able to add the tags, but maybe the bookmarklet is capable of that?)

I remove the bookmark's name so only the Pocket icon shows. It's very slim, but you do need the bookmark bar for that. I suppose it would work from the bookmarks menu too.

When you click the bookmarklet a little dropdown banner shows and you can add tags from there.

You might like the "Custom Buttons Bookmarklet Maker" I use it to have a button for adding videos from a page to my Plex queue, removes the need to have the bookmark bar on screen. I found it rather arcane to set up however.

https://addons.mozilla.org/en-US/firefox/addon/custom-button... ... ah, and now I see it's unmaintained and recent version isn't working for some, carry on, never mind (works for me though).