Why uploading large files works in Portable Firefox, but gives warnings Mozilla.org Firefox?

Hi,
in Mozilla.org Firefox (Finnish version) when I upload
large files (2-6 Mb) in my SquirrelMail webmail form, in middle
of transfer it gives trasfer warning (translated to English):

"Warning: commandfile does not answer to calls.
- In this page commandfile is busy or does not
answer to calls. You can stop executing commandfile
execution or wait for it to finish.
[Stop commandfile] [Continue]"

If I click Continue, it asks the same question again many
times, until file uploading is finished.

But, in Portable Firefox, it uploads file
without warnings, straing to finish! Why? What setting does this?
I tried to change about:config setting in Mozilla.org Firefox
but it still gives warnings.