I still customise my Chrome and Firefox mouse gestures add-ons to work exactly this way, despite having long forgotten where those conventions originally came from.
Other gestures I have, probably also from Opera:
DL: Left tab
RL: Right tab
D: Page Down
U: Page Up
RLR: Close tab/window
Sadly the last two were killed off in Firefox in that last big add-on update. I didn't find anything that works since then.
Sure, my current browser supports the latest web standards that enable great things and sure, my internet speed is a lot higher. But just from a "browsing the web" perspective, Opera was the high point.
Though, Opera tried many things in that area. There never really lost the MDI-Spirit and went with different approached than most other TDI-Implementations. Made it more useful for some, more complicated for others. So for certain specific functions they probablly were the first ones.
However, Opera supported an MDI interface before it supported tabs, and if you count that it may have supported multiple browser windows within one MDI window even earlier, which could be said to be a form of tabs.
Everything depends on how you define "tabs", as usual.