I am reporting this in Firefox 7, but I have seen it happen in previous versions too. 1) Open Firefox 2) Open preferences 3) Click content tab 4) Click "choose" on the languages at the bottom 5) Choose a language further down in your language list and click "move up". Expected: The language should continue to move up the list after every click of the "move up" button until it reached the top of the list. Actual: The language moves up once or twice and then fails to move up any further and starts swapping position above. The only way I can get a language to the top of the list is to choose another language and then choose the one I want to move again and it may continue to move up the list. This does not happen at all with the "move down" button as it does exactly as expected.
This works for me in 15.0a1 (2012-05-15). Reporter: do you still see the issue in the latest build?