I use a simple bookmarklet for Pocket. This consumes zero resources and works just fine.
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?)
When you click the bookmarklet a little dropdown banner shows and you can add tags from there.
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).