Hi   ! I'm sorrry, but Firefox doesn't do email; it's strictly a web browser. If you are using Firefox to access your mail, you are using 'web-mail' like Yahoo!Mail or Gmail. You need to seek support from your service provider or a forum for that service. If your problem is with Thunderbird, let us know and we can move this thread to the Thunderbird support queue.
For further information see :
'Problems with email and how to find help'     http://mzl.la/1xKvOMQ

I don't think Happy112 is reading MozAddicted's statement carefully enough; MozAddicted carefully lists the names of two email services -- gMail and Yahoo Mail -- impying that if he tries to use Firefox as his browser for those servcies (not Chrome, or IE, or another browser), his problem occurs. That is my experience, too. But it only started about a week ago or less. I'm fine when I attach a file to a message, but only if I am using either gMail or Internet Explorer... Here's the problem, in a truth table: Attaching a file, using this eMail Service Provider: Browser : gMail Yahoo Mail Firefox Fails Fails Chrome Attaches Attaches I E 11 Attaches Attaches

(I have used Firefox for at least five years, and this has never happened, before about a week ago. )

That describes "Layer One" of the problem... Here comes "Layer Two":

About a week ago, as the global community was reeling from the ransomware attacks in UK and Europe, Verizon, my ISP, switched subscribers into McAfee-based "Verizon Internet Security Suite Multi-Base" My internet experience didn't suffer from that change -- except for the possibility that the change somehow might have caused some default or customization setting somewhere in my PC to malfunction.

Perhaps the recent Firefox update to 53.0.3 is the perp...  ? ... or perhaps one of the 7 Microsoft's updates I received last week???

But why doesn't this problem occur when I use a different browser?? Doesn't that show that the change in the network security and the updates to Windows are probably not the causes, and that it's something in Firefox?

Thanks in advance for your help!

JES08 Windows 7 Home Premium SP1 Firefox 53.0.3 IE 11 Chrome 58.0

Apart from the fact that the OP posted his question over five months
ago and never replied :
Two things you could do : Start your own thread   (copy and paste what you posted here,
if you like).
Or you could read the article that the OP apparantly did read :
Problems with email and how to find help