I've been having this issue with Firefox Developer Edition (FDE) on Youtube, where it show videos only in 360p quality, there's also an "Auto" option, which is also, the 360p quality. I'm in Ubuntu 14.04 LTS.
Here's an example of a video that I know is in higher quality because I can see it with Chromium and with youtube-dl.
On Chromium and youtube-dl:
What could be the cause of this?
I have installed the Adobe flash plugin and I keep all my packages up to date, including my version of FDE (40.0a2, updated 18th may 2015).
I want you to know that "switching from FDE to Chromium" is not the solution to this question and I will not mark those posts as the answer.
I tried deleting every Youtube and Google account related cookies but it didn't solve the issue.