When using Mac and Firefox, why can't I get a written web address as a link when...

[steve c]

...I write in a yahoo mail.? Using explorer on a PC and its OK.
On the mac with firefox it doesn't come out as a blue underlined link.
e.g. www.blah.com or http://www.blah.com

[I tell it like I see it]

did you hit return after the link? sometimes that's all it needs to become an active link