I'd like to upgrade Firefox to the new version but I don't know if I'll still be able to read pdf on it with this this plugin. Is there another way ?
The PDF Browser Plugin from Schubert|it recently added support for Firefox. It's free for personal, non-commercial use. I've found it to be a very capable in-browser PDF viewer.
I've looked for a long time, and couldn't really find anything. I use the Open IT Online addon now instead—I like that it is integrated into the Download file dialog and gives an option to Open IT Online in addition to saving and opening in Preview. It supports opening in Google Docs, Zoho, and a few other services.
You can use Adobe Acrobat, in the preferences you can set to open in web or open in Acrobat.
If you choose web, it will open in FireFox
If you choose Acrobat, it will download the .pf and open in Acrobat
This is currently the way I will do it as Firefox has gone from Firefox 4 to FireFox 10 beta. The reason is to catch up with IE in releases. (apparently)