Thank you so much for reviving the old pocket addon for Firefox.
Unfortunately Pocket(or should I say firefox) has removed the ability to search by tags in the free version.


Update #1
I got reply from Pocket team regarding the tags issue. I think the reason below is why you can't implement tag search option. Hope this helps.
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Thanks for contacting Pocket Support, Rajitha! I'd be happy to explain a bit more.

In the free version of Pocket you can only use our Search tool to find items by URL or title. We do offer a way to search Tags unless you use the full list.

You will need to look at the Tag list to search by a tag in the free version.

However, our Premium feature does offer Tag search in the Search Bar. You can learn more about our Full-Text Search Premium feature by following this link: https://help.getpocket.com/article/894-pocket-premium-full-text-search

Please let me know if you have any questions on this!