This article deals with websites not appearing correctly in Firefox.
Firefox caches websites, which means it saves some files on your machine so that it doesn't have to re-download everything when you visit sites. If a website has updated their code, Firefox may still be using some of the old code along with some of the new, which can make a web page display incorrectly. To fix this, you need to clear the cookies and cache:
Some websites may not look right at different zoom levels. To reset Zoom setting, for the site, press Ctrl + 0command + 0 and for all sites, see the documentation about the Zoom add-on you use.
Some sites do not display properly with a minimum font size. To reset the minimum font size:
You may have inadvertently set the page style to No Style. To ensure Firefox is set to use the page's default style:
Now that the page is using its default style, it may be displayed correctly.
In some instances the loading and verification of secure web content will be time-dependent, so you should check that date, time and timezone are set correctly on your system.
Some extensions may interfere with the way websites are displayed or your graphics card and drivers don't display some web content correctly when hardware acceleration is turned on. Follow the instructions in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to see if one of these is causing the problem.
Occasionally you may come across a page that simply doesn't work in Firefox. If that happens, please report the problem to Mozilla as follows:
Based on information from Websites look wrong (mozillaZine KB)