I moved from IE a couple weeks ago and about the only problem I'm having is that Firefox appears to be deleting cookies after a couple days. For instance, I visit several forums atleast once a day that need login info for entry. In IE I could go months without having to type in login info because the site got the info from a cookie. But in Firefox every couple days I need to manually login.
I don't see anything in options that could be handicapping the cookies, so I don't know what's up.
Netscape has set a 300 limit on cookies and deletes the least recently used cookie when the limit is exceeded ( Netscape Reference [devedge.netscape.com] ) So it makes sense that the same limit appears in any Mozilla browser.
Does that account for what you are seeing - a number of the sites you visit regularly are placing boatloads of cookies (near the limit of 20 per server), or you visit a boatload of sites that place cookies.