I tried to edit my post and got 500 errors. But when I returned I saw that the margin on the header has shifted over to the right with all the browsers (ie, Opera and Firefox) for the forum index page and the thread index page. This maybe a fixed width problem.
I checked my saved (off line) pages and checked them against different resolutions. I think (this is the reason I asked about what software you are using) that the changes are not allowing the browser to fill in the margins like the old software used to do. I find this problem in the recent ecommerce suite upgrades that i used, they upgraded the software and it messed up all the stores margins.
Not to criticize you or your web developers but I have a suggestion for future migration and upgrade. You should make all the changes on another server, if not at least a sub-domain, have all the testing done there and even invite people to look at the changes and make suggestions before going live. I only say this with years of experience in IT/software/web development.
ORIGINAL REPLY
Firefox, which I understand that this may be the problem with margins, but Opera and IE have a similar issue. looking at one old page and the new one, the margins are just a little too wide on the sides.
Maybe I am too picky about web sites.
Oh I am running 1280 X 800 which i know will affect things.