Having same problem - use a dual boot and have it with both Windows 7 and FF 18, and Windows 8 and FF 19. Since 2 days. No issues with either system using IE 10 or Chrome 24. To be fair, running some addons like No Script and Ad Blocker Plus and Ghostery. But these have worked fine allowing various exceptions for months.

Have cleared cookies and caches-no change.

Something changed 2 days ago and of course it could also be at the yahoo mail side. Big annoyance as I now have to load other browsers just to answer mail.

Suggestions? Anyone else (seems so) having the problem.??? Thanks

hello esysfound, can you try to replicate this behaviour when you launch firefox in safe mode once (in order to rule out the interference of any extension)? thanks...

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

Hi- good idea. So it worked - and it suggests an add-on, or yahoo mail is the culpret. But-all was fine til about 2 days ago, and that the problem occurs with 2 OS's and 2 different versions of FF (same add-ons) does not really narrow it narrow it down. I know No Scripts updated today, and Ghostery 2 days back (that might be where to start) - will try disabling. Except again these were working fine, really useful) and no way to know if the problem is there or at yahoo end.

OK- more tweaks. It does appear to be Ghostery-will work on that. Thanks for the suggestion-should have thought of it myself.

Not sure if this helps others but it does seem to be bothering a number of people.

yes, thanks for reporting your findings!

Hi- well it turns out to be a little more complicated - I run a Ghostery add on in Chrome as well, tested it, and that works fine!!??

So there is some interaction going on w FF. What I finally did was white list a few yahoo sites in FF Ghostery options and that has solved it. Still do love FF though it is work sometimes. Again thanks for your suggestion which helped in the detective work. Now-back to real life...

So, I figured out that some time on Thursday, 17 January (for me, at least) Yahoo! email began disallowing those of us who choose to use Mozilla Firefox (17 or 18) with tracker blockers like Ghostery and AdBlock Plus's Fanboy's Tracking List from sending outgoing email. Any email to which I replied was populated with the phrase "Yahoo! Mail found the original email, but encountered problems displaying it. Please try again later." and I could not even address, let alone send, a new email.

I'm not yet sure which particular trackers I needed to allow so that I can send outgoing email, but I do know that if I whitelist "mail.yahoo.com" in Ghostery and turn off AdBlock Plus's Fanboy's Tracking List, then I can again send email via Yahoo! email. Probably ScoreCard Research and Right Media need to be allowed, since those pop up in every Yahoo! email, but I'm sure there are others with which Yahoo! wants to track their users.

I'm not sure why they would do this, other than the obvious, to track their users. But there are repercussions that will cost them. I was happy to see SSL implemented recently on Yahoo! email, but after this move, they've made me search for a new, reliable, secure email service for the first time since the 90s, and when I find one (suggestions?), I'll never think about Yahoo! again.

So. It would appear that if you wish to continue to use yahoo as your primary email AND block them from reading and tracking you emails then you will have to configure "Thunderbird" to retrieve and send your email. It appears to work just fine and dont think anyone else will ever know the difference. Did it earlier and seems to be working great for the moment. Will update as time goes on if something shows up that is not working. Good luck all.

If you go to the Yahoo mail page & turn off Java in options the page asks if you want to return to the previous version of Yahoo mail. Confirm (always - not once-only) & the old version appears. Turn Java back on & you'll have your old Yahoo functionality back without screwing with your Firefox settings.

Thanks but not really a solution. I want the new version of Yahoo, and this does not address the problem, other than the old way does not send some javascript.

Something changed at yahoo end that means you cannot send mail unless you allow certain trackers.

Anyway I am done with this as I have my workaround. Thanks to all for the help.

I found that the problem for me was having my AVG toolbar enabled. I suspected that might be the problem since AVG recently had me update the toolbar. I disable it to send an email and enable it the rest of the time. It is a pain, but significantly less of a pain than not knowing the issue.

UPDATE: FIXED. Madman's response about hitting the link to this other website worked for me:

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

My problem ended up being the AVG Toolbar Extension - disabling it fixed the problem!!!!

(I run Win 8 and Firefox 18 and the current Yahoo mail. I don't see anything under options about Java. Please help me find this option.)

Madman's response about hitting the link to this other website worked for me:

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

My problem ended up being the AVG Toolbar Extension - disabling it fixed the problem!!!!