Among the developer highlights for Firefox 26 are support for new CSS properties, continued ECMAScript 6 support with new mathematical methods, API improvements, support for VTTCue, and the Firefox Inspector is now remote accessible over the network. The developer-focused changes for Firefox 26 can be found via the (tentative) developer release notes.
On the end-user side, the Linux version of Firefox 26 supports H.264 video codec support in conjunction with the appropriate GStreamer plug-ins.
Firefox 26 also defaults non-Flash plug-ins to now working in a "click to play" mode, the password maanger now supports script-generated password fields, and improved page load times by not decoding images that aren't visible.
Firefox 26 doesn't include the VP9 video codec support but that will come in a later Firefox release. Also not yet found inside Firefox officially is the Australis user-interface.
While we're still waiting on the official Mozilla Firefox 26.0 release announcement, it's happening today and the binaries have already been uploaded for your consumption now via the Mozilla.org FTP server.