Scroll down for the reports that this is happening. Have identical results where it goes to 540p after watching 1 video. (EDIT: This issue is specific to that addon)
(that is, if they encoded in 540p at all, because they usually only encode up to 480p for the SD resolution.)
Reports are that Chrome and Chromium are locked to SD and Firefox Vanilla and ESR without this addon can get 720p.
Confirmed with a Windows machine that it is not restricted to 480p or 540p on any browser that is up to date.