Not quite sure I understand your problem, but is it that you want to copy the address for the old thread and paste it into a new one? If so, first question is do you see the address bar at the top of your browser...it'll say "http://davesgarden.com/something" If you see that, you should just have to highlight the text in the address bar with your mouse, then hit control-c, then go to the new thread and hit control-v in the box where you're typing your post.
If the address bar is missing, at least in Firefox for Mac if you go to the menu and select "view", then "toolbars", there's one called navigation toolbar. Make sure to select it--if it accidentally got unchecked then you won't see the address.
If none of this helps, it might help if you could describe in a bit more detail exactly what exactly you're trying to do, how you used to do it when it was working, and exactly which steps aren't working for you now.
I am also confused as to what you want. How about Ctrl + T to open a new tab next to the old one. Use Ctrl + C to copy the link in the old tab and Ctrl + V to paste it in the new tab. Now you have two tabs with the two most recent links.
Where exactly are you right-clicking? And do you get any menu options at all when you right click just not the copy/paste that you're looking for, or do you get nothing at all? Might also help to know what version of Firefox you're running (and I assume since you're talking about right clicking that you're on a PC not a Mac), that way maybe someone else who has the same browser can try to replicate what you're doing. And are you having trouble with right clicking not working properly in other programs (which would suggest it's an issue with your mouse settings), or is it just Firefox (suggesting it's likely a browser issue)? FWIW, I'm running Firefox 16.0.2 on my Mac, and if I do the Mac equivalent of a right-click when the cursor is in the address bar, it does show an option to copy.
In the meantime while you wait for someone to maybe figure out why your right click doesn't work, if you do what PotEmUp and I suggested (highlight the text in the address bar, then ctrl-c to copy it and ctrl-v to paste it into your new thread) that will do the same thing. Might take an extra fraction of a second longer than right clicking, but it accomplishes the same goal.
Thank you ecrane3, for not giving up on me. lol For some reason, the upper portion of the screen will not show up until I move my mouse up top. Once I did that I was able to right click and copy and paste as I usually have done many times before. It's a learning curve for me.