Looks like the page isn't rendering correctly. Let's update everything first and then start looking for culprits if necessary; your question's details say you're on 45.0 (45.0.2 recently came out for your OS) and your Java is out of date.

1. Update Firefox. Either use automation or visit https://www.mozilla.org/en-US/firefox/all/.
2. Visit https://www.mozilla.org/en-US/plugincheck/ to check your Plugins (Extensions should update automatically, but you can make a manual check by going to about:addons, selecting the Extensions tab and selecting Check for Updates from the gear icon menu in the top right corner.

PS. Here is the general support article that contains some rudimentary steps you can try: https://support.mozilla.org/en-US/kb/websites-look-wrong-or-appear-differently.

If you are feeling nerdy, you could try Firefox's Inspector tool. Right-click the blank area and choose Inspect Element (Q) from the menu. Firefox should open the Inspector (usually in the lower part of the tab) with the clicked element selected in an HTML tree diagram. Does it look like maybe an ad that's being blocked, or ??

Some information about mixed content on web pages:

You can see a special padlock at the left end of the location/address bar.

• a padlock with a strike through means that mixed active content is blocked.
• a padlock with an exclamation mark attached means that mixed passive content (e.g. images) is present, but not blocked.

The shield icon, previously used for mixed active content, is now used for Tracking Protection.

• https://support.mozilla.org/kb/mixed-content-blocking-firefox
• https://support.mozilla.org/kb/control-center-site-privacy-and-security-firefox

• https://developer.mozilla.org/Security/MixedContent

I still only have 2/3 of a screen to process my emails!

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: "3-bar" menu button or Tools > Add-ons > Appearance
• Do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History").

Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks and history and cookies and passwords and cache and exceptions, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.