Is that Firefox?
For Haiku themes on linux it is kind of sad that KFE4 and KDE4 actually had window tabs, but they never reimplemented them into plasma (it was in their missing features section for a long time, before they removed that)
Ocasionally I see users on random forums say kde should add window tabs as a new innovative feature, collective memory is short i guess.
I assume the E is for enlightenment, do they have window tabs? I can’t really recall since it has been a long time since I’ve used it (input latency there has always been a killer for me : / )