Firefox for Android gained support for several languages spoken in India as well as support for WebVTT (subtitles) and CSS3 variables. This app also lost support for Android devices running Android 2.2 Froyo, but on the plus side users can now also refresh synced tabs on demand.
According to the changelog the new version of Chrome for Android benefited from bug fixes and performance improvements, and it is also supposed to have better support for rendering text on non-mobile optimized sites. To be honest I am not sure I can see a difference.
The Dolphin web browser, on the other hand, is showing more visible signs of improvement. It too gained support for additional languages (Russian, Arabic, Brazilian Portuguese, Bahasa Indonesian, and Hindi). There's also mention of a Gmail and Android L bug fixes, and most importantly Jetpack has been integrated into Dolphin on Android 4.1.
This last is an add-on app which is supposed to improve performance. I say "supposed to" because I've usually had to go in to the settings menu and make sure that it was disabled. Twice now it has caused more trouble than it has helped.