But when I put mark forums read then eventually close the browser then I come back to the forum it marks threads not read yet. I always thought it was just my work computer but now at home It is starting to happen. I use IE at works and firefox at home.

maybe

Are you sure that the threads didn't have a new post in them? I do the same thing as you. I open all the threads I want to read in different tabs and mark the forums as read. Havent had any problems like what your describing.

nope it just happened again with no new post in them.

do you clear your cache every day? also, when you mark forums read do you get the pink bar across the top that says "messages read"