I use Mozilla Firefox ESR 52.6.0 and Mozilla Seamonkey 2.49.2 on Ubuntu 16.04.4 LTS x86_64 and Windows 8.1 x86_64 with installed updates.
After some updates of site design its layout is somehow broken in the end of post (between end of question and comments).
Screenshot of fresh question with long content (only problematic part, I'm not logged in, 100% zoom):
Screenshot of other question viewed from Firefox (only problematic part, I'm logged in, 30% zoom):
Both questions looks normal on Google Chrome 64.0.3282.186 (Official Build) (64-bit) in incognito mode without addons.
Where should I report this problem?