Good work checking cache and cookies. Next, can you rule out any content blockers as an issue? Those might include:

(1) Built-in Tracking Protection feature (especially in private windows).

Do you see a shield icon toward the left end of the toolbar when this problem hits? More info on how to use the shield to manage blocking: What happened to Tracking Protection?.

(2) Content blocking extensions such as Adblock Plus, Blur, Disconnect, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix.

These typically provide toolbar buttons to manage blocked content in a page.

Thanks for feedback.. I don't have any built-in tracking protection as far as I know.

No shields appearing on the toolbar toward the left.

Re. Adblock suggestion, I was unable to figure out toolbar buttons.

Do you notice any document types working better or worse, or are all attachments affected equally?

Could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

Problem turned out to be running Adblock and Ublock concurrently. Removed Ublock.

Thanks for the suggestions.