Steps to solve this:

• Cookies and cache should be cleared.-Firefox caches websites, which means it keeps some files on your computer so you don't have to re-download everything every time you visit a website. If a website's code has been updated, Firefox may still be utilising some of the old code alongside the new, causing a website to show erroneously. You'll need to erase your cookies and cache to fix this.
• Reset the zoom level-Some webpages may not appear correctly at various zoom levels. To reset your Zoom settings, click Ctrl + 0 for the site and consult the documentation for the Zoom add-on you're using for all sites.
• Reset the font size to its smallest setting.-With a minimal font size, some websites will not appear properly. To restore the minimum font size, follow these steps: Select Settings from the menu button. Navigate to Language and Appearance in the General panel. Click Advanced in the Fonts & Colors section. Set the font size to None as the minimum.
• Reset the page's appearance.-It's possible that you set the page style to No Style by accident. To make sure Firefox is using the page's default style, do the following:To bring up the conventional Firefox menus, use the Alt key, then click the View menu, then select Page Style, then Basic Page Style.

Read More : Why is Firefox not loading pages?