2 ways. Easiest...Bookmark this to use to enter FlyerTalk:

http://www.flyertalk.com/forum/

...and each threads URL will show up in your browser's address window.

If you insist on using www.flyertalk.com which gives you the tabbed frames, then when you are on the thread you want the URL of, just Right-Click and select Properties. It will show the thread URL there too.