Make sure you are not blocking content.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Sorry about that. If you adjust the zoom one tick -- hold down the Ctrl key and tap either the + or - key -- does the missing content fills in?

Your symptoms plus the zoom test are characteristic of a new bug in Firefox 59-60 affecting users who disabled ClearType font smoothing in their Windows settings.

If you don't want to re-enable ClearType, then the workaround is to temporarily disable Firefox from using hardware acceleration. The following article has the steps for that, and the change takes effect the next time you exit Firefox and start it up again:

Firefox's performance settings

Any improvement?

FredMcD said

Make sure you are not blocking content. Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

Thank you kind sir, this has worked for me.

It could be the work of one of your add-ons, or it could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All Of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.