Why can't I log into certain sites on my mac, but its fine on PC? Change MTU?

Certain sites I have to log in to will not work on my mac, but I can get into them on my PC. Specifically yahoo mail and facebook. Yahoo keeps asking me to verify my password, but it doesn't work. Facebook won't let me in at all. I am running OSX 10.5.8. I have researched the issue a bunch and found similar issues from others. The solution that seemed to work for them was manually changing the MTU setting to 1400. I have tried that for the airport setting, but it keeps defaulting back to configure automatically. 1) I don't even now if this is the actual solution, so can you advise? 2) How do I get the new manual MTU setting to stick? FYI, I have enabled cookies, deleted all the current cookies... basically done everything with cookies I can. I don't think that is the solution. If you don't have the actual answer, can you tell me what I good MAC chat forum is where I could post the problem? This is getting really annoying. Thanks!