So, to get this straight...
The multiple attachments did not work at all until you switched to Mozilla? If that is the case I will let the Support Team know so they can advise others who may come across this problem.
I have reduced the attachment allowance back to 100kb per attachment, but increased the maximum to 400kb per post, where before it was only 300.
This means that from now on everyone should be able to post 4 attachments at no more than 100kb (each attachment) on each individual post. If, somehow, you receive an error message again, it will probably be because the file sizes are too close to the allowed limit.
I'm really glad that Mozilla is working for you, MBL, and that you weren't averse to testing it. Thanks for your patience.