If you want to log into multiple ebay accounts on firefox for shill bidding / selling etc, it is simple.
Just create multiple profiles, the example below is for Linux
Copy these instructions before going any further.
In the command line type "firefox -P"
Click "add profile" and call it "default2", don't overwrite your existing profile or your bookmarks will be lost. Create as many profiles as you need!
Close off the profile manager
Now you can create a shortcut for you new profile
"firefox -no-remote -P default2"
So your original firefox shortcut will open the "default" firefox profile and your new shortcut will open the profile called "default2", notice that profile default2 has no bookmarks etc.
You can now change the appearance / settings of the profile default2 to match whatever. I made mine look like a different colour.
Works for multiple gmail accounts, twitter, facebook the list goes on and on. It will never work for accounts that rely on IP addresses like file download sites i.e oron or rapidshare.