is the X11 mouse insert buffer limited to some amount of KB or
characters by Mozilla 1.6?

When I have lots of text to copy and paste, Mozilla 1.6 usually does not
insert anything. The mouse buffer's content is indeed inserted by Emacs.

I have a text file of 834 bytes containing 62 lines like this:
24.02.04 213
In a 80x60 winterm, I "cat" it's content seven times on the shell and
mark the whole output. Mozilla does not insert the mouse buffer into a
"compose new mail"-window. Emacs has no problem at all. Mozilla does
insert the output of three cats but it refuses even at four cats.