Hi!
I've tried to upload books but when I attach them and then try to post, it doesn't work and Firefox indicates that the connection has been reinitialized. Has this happened to someone else? Btw, the file attached was a book with a correct size, and is one of the allowed file types. I tried with other books but still didn't work.

Megrose is right. You can redownload v7 here:
http://www.filehippo.com/download_firefox/10687/
Thanks to greyfox for the link. 
I also use Chrome. I switched from Firefox because of problems like this.

I run Firefox and have (re-)upgraded to Firefox 8.0 and have not had any problems with uploads. It's most likely your internet connection- the connection is timing out when trying to upload the files. Do you run wifi? If so, you could try connecting via ethernet cable and try again. If not and you have a separate computer, try it from a different one. Also, if you are uploading multiple files at once (or large files) it might time out...you could try uploading individually (try the smallest file size first).

do you have a secondary computer you can try? if not, and you have access and a laptop, you could try taking your laptop to an open wifi spot (for example: McDonalds, a motel offering wifi, etc) and connect then try uploading.... if not, and you have the money, call your internet service provider and ask them to upgrade your speed.