It's kinda nuanced. Opera devs in late 90's and early 00's were truly innovative. Tabs is just one of the things introduced by Opera and later adopted by every

It's kinda nuanced. Opera devs in late 90's and early 00's were truly innovative. Tabs is just one of the things introduced by Opera and later adopted by every other browser. But they also introduced a lot of under the hood tricks to render pages on slow connections, delay loading images etc. But their problem was in quality. Partially it was MS with IE dominance to blame, but anyway, html rendering by Opera was inconsistent. So as a paid browser they failed as soon as 33600 modems turned history and Firefox introduced most of Opera features for free with much more consistent page rendering.