It is possible that the link in no longer valid and you get redirected. You can compare the link in the bookmark with the link that you see on the address bar. A possible cause is a protocol mismatch where your bookmark has the http:// protocol and the website forces the https:// protocol.

Sorry ,perhaps I did not explain properly.ANY link in the bookmarks do not stay bold/highlighted when clicked,this becomes a pain as I have several bookmarked threads from the same group and have to try and remember which link I am on when I scroll to the next page ,or I have to scroll to the top to remind me which thread. I have cured by finding the old profile chrome folder with the css file,however I would have expected this feature in ALL firefox versions.