Instapaper has a free plan. Personally, I moved away from Instapaper and use the extension MarkDownload to save pages as Markdown and import that into Obsidian.

The Supermassive Podcast by the Royal Astronomical Society is very good: https://podcasts.apple.com/us/podcast/the-supermassive-podcast/id1495324190

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?