Firefox for Android has a rather minimal design. Organizing bookmarks is a complicated feature set. Syncing is the current way to achieve what you want to accomplish.

There are many reasons why threads get archived. Threads that have not been commented on for a long time are archived. This is done to avoid the second reason. Sometimes a forum user will bump a thread with a post that is off-topic or spam, that is another reason to archive a thread. There are occasions where people on the thread are choosing to behave poorly. There are probably other reasons but those are the ones I can think of. Starting a new forum thread is inexpensive and can avoid outdated information being perpetuated.