hi, could you try to replicate this behaviour when you launch firefox in safe mode once? (this is just for testing purposes and not a general solution)

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

The problem does not occur when FF is launched in safe mode. I've just updated my extensions, restarted FF, and the problem remains. I should add, my 'Latest Headlines' bookmark was working fine with these same extensions prior to updating to 39.0 Beta 5.

could you try narrowing it down to the extension which is interfering here, by disabling them one-by-one/in batches depending on how many you have? afterwards it would probably be helpful to inform the addon developer about this...

Right, having repeated the process several times, I can say that it's the HTTPS Everywhere extension causing the problem.

ok thanks, so this probably isn't a general conflict with live bookmarks and https everywhere but only with this particular rss feed which gets "upgraded" to https but doesn't work for some reason. can you create an exception for that one page in the https-everywhere settings?

I'm reading up on that at the homepage but it's not so easy, it seems to involve writing an XML file....

I suppose for the moment we can consider this issue settled- the problem has been isolated, though not fixed- and I'll report back if I come up with a solution.

Thank you so much for your time Phillipp, you've been most helpful.