What do you mean with "merging and splitting tabs"? for Firefox I use the good old FireGestures https://addons.mozilla.org/en-US/firefox/addon/6366 For Chrome I used Mouse Stroke https://chrome.google.com/extensions/detail/aeaoofnhgocdbnbe..., but I will give "Smooth Gestures" a try.

Update: "Smooth Gestures" are better than "Mouse Stroke", thanks a lot!

PS: You can not possibly make an "productivity addons" collection without Mouse gestures, they increase your browsing speed easily by 100%.