I think that these three suggestions would be a good place to start:

• Clear your cache in about:preferences#privacy
• Launch in safe mode in about:support
• Make sure hardware acceleration is enabled in about:preferences

If those fail, it might be handy to copy the info from about:support to have a look at it.

Zack: First 2 made no difference, as far as I could tell. Hardware acceleration is enabled.

Problem happens often but will go away for a while, without my doing anything, then returns later. That makes it hard to tell if anything I've done has reduced it. It still occurs fairly often, whatever I do.

I've observed something more since my initial report: If I just leave the error message on screen for a while, the error message goes away & the video comes up, ready to play. This suggests to me that something is getting its timing wrong/being slow to start but eventually catches up. (Time required varies from ~20 to ~40 seconds. As I know I can get to the video by pressing Ctrl+R as soon as I see the error message, I don't usually have the patience to wait for it to correct itself.)

Note: Since Ctrl+R gets me what I want, this problem is merely a nuisance & not worth major effort to solve. It's puzzling, though, & I was hoping someone knew the cause.

If you suspect that something is being blocked, you can check what is being loaded with the web console:

• https://developer.mozilla.org/en-US/docs/Tools/Web_Console

If all else fails, you may try refreshing firefox in about:support

Some other things I may suggest:

1. Firefox uses openh264 to decode video formats, check to see if it's loaded properly in about:plugins
2. It still may be handy to paste the text from about:support so we could analyze it

Is this unique to firefox?

Any possibility this is caused by an ad blocker?

jscher2000: I do use AdBlock Plus but that's been in place for years. It hasn't caused problems before. Besides, running Firefox in Safe Mode (no add-ins) should have removed the problem if ABP were the culprit.