Since I updated from 1.5.4 to 1.5.10 I get reports that visitors of my site see a horizontal scrollbar in the text editor (see attachment). The problem only occurs in older Firefox versions (e.g. 28.0).
I don't think we've actually changed anything here, but I would say that we won't really support old Firefox (or Chrome, for example) releases. Since moving to automated updates, we're only likely to support very recent versions.