I used the instructions linked below, and was able to watch xfinity on Ubuntu 14.04 in Firefox by using WINE and the PlayOnLinux (the easy to use front-end for WINE) available at the Ubuntu Software Center:
Cannot Play DRM-protected Flash on Ubuntu 14.10 (Amazon, HBO Go, XFINITY etc), even with Pepper Flash and HAL
I did not need to install Silverlight, but I did install the Adobe Flash add-on, which was available during the installation process for Firefox. Adobe Flash version 15.0 was installed, and that solves the issues of watching xfinity in Firefox on Ubuntu.
I "downloaded" and installed Firefox through the PlayOnLinux front-end when given the "Choose and installation method" option. It is listed when you click on the "Internet" icon. Make sure you DO NOT run Firefox at the end of its installation procedure, i.e Deselect "Launch Firefox now." If you don't deselect it, Firefox won't install properly.
Information and helpful Manual on how to use PlayOnLinux can be found here:
When Firefox is installed, it will show up in the PlayOnLinux program window pane. You can then lauch Firefox from there (or the desktop shortcut that may have been installed during the procedure), and enjoy watching xfinity.
Note: I updated from the downloaded FF ver. 31 referenced above, to the latest ver. 34. That caused some problems in the Menu Bar. For some reason, clicking on a menu like "Tools" will display the drop-down menu, but clicking on items within the menu sometimes displays only a partial window, where the other part of the window is black. I have an old computer, so the problem may be related to my system. YMMV