I can’t recommend Musicbee enough: https://www.getmusicbee.com/. It’s completely free, extremely customizable, updated consistently, has healthy ecosystem of plugins, and has a helpful community if you have questions about using any of its features.

Mediamonkey is also a good choice, but once I found Musicbee I transitioned and never looked back: https://www.mediamonkey.com/.

Edit: I forgot to add that both options support playlist and visualizers. Musicbee even has smart playlists that can update automatically based on custom rules you set up. (If you want that feature in Mediamonkey, then you have to pay for a license.)

Rather than try and force Firefox to deal with thousands of tabs, it’d be easier to use an add-on like SingleFile to download the tabs as self-contained HTML files. After that, you can search their contents using free tools like Agent Ransack or DocFetcher.

If you prefer to keep the data in your browser, then how about using a service like Instapaper that lets you save pages for reading/referencing later as well as search their contents?