Whether I view the desktop or mobile version, I have a "bookmark topic" button above and below the
thread. (I'm on my phone.) If you'd like to see the desktop version on your phone, there are a number of ways. The first several I tried were unreliable, at least on my phone. Right now, I'm using xbrowser, with a greasemonkey script called "Refuse mobile page!".
Or if you'd like to work with your current browser and view, can you take and post a screenshot from the top of your thread? I'll attach one of what I see.