There should be an option (next to follow discussion) when viewing topics to add it to a new “read later” list.
You could then open your read later list on the bottom of the “Discussion Forums” main page, next to the “your topics/posts” links.

I’ve often found I want to reply to a topic, but don’t have the time to write a full response.
A “read later” list would let you open these topics a little later when you are able to respond.

Q: Why not just remember the name of the topic and search for it again?
A: There are a lot of posts I want to reply to, so I can’t remember all of the names of the topics.
Q: Why not follow the discussion?
A: I forget to view it later on.

\(-_-) ::#00AAAA //This is crypto. He protects my signature from the evil kumquats!

bookmark it instead, aka:
CTRL+D on Firefox/Gecko-based browsers
CTRL+B on Chrome/Chromium-based browsers

sort them in a seperate folder if you need.

EDIT: Or store the URL somewhere, write it down.