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Mozilla Firefox is Working on a Tab Grouping Feature
(news.itsfoss.com)
A place to discuss the news and latest developments on the open-source browser Firefox
I'm away from my PC so I can't name it but there's another plugin similar to Tree Style Tabs. The creator claims that TST takes up a lot of resources. I do notice Firefox taking a lot of CPU/mem but that's probably my fault. I've tried both and either works well.
Might be talking about Sidebery. I like them both and switch back and forth