Setting Thunderbird message Length

[GaryT]

Fred Roller wrote:

>> [BIG SNIP]
> Do you have an alternate smtp server you can try.  Just to eliminate
> that option and put the problem in your system?

Thanks, Fred.  Yes, have done.  The details of those attempts are almost
unbelievable.  Two separate and unconnected WebMail servers - both same
negative result until now it's worse, I'm unable to access the WebMail 
of my ISP.

Signd in and displayed the Account page, seleted WebMail, browser 
reports "Firefox prevented this page from automatically reloading".
Now no WebMail - not that I use it.

Meanwhile I had already spent an hour in each of 2 separate sessions
with Tech Support confirming that the problem appeared to be on this
machine.
No further help there - ISP supports only Microsoft products.

When this all began I was unable to attach a small file to a Thunderbird
message - then "message length" itself was affected - this message was 
far too long & severely chopped.

Perhaps there is a little known Mozilla setting somewhere in the depths.
Or a malicious file that somehow got past FireStarter.
And why the FireFox message, when the WebMail page was loading?
GT