On our project site, with Firefox 4 and 5, buttons don't take the keyboard focus if you mouse-click on them. Is this a problem of our site?

On a page like http://www.ori.uzh.ch/links.html, clicking a link opens a new tab, and after closing that tab, you can move to the next link with the tabulator key. On that site, it still works with Firefox 5. I also worked on our project site (password secured, sorry) with Firefox up to 3.6.18. But now, if I open a link with the mouse, close that tab again and press the tab key, the focus just goes to the first clickable button, as if I had not clicked any button previously. If I open a link by tab key and "enter", the keyboard focus is preserved on that button. But there are many many buttons on our pages, and they reload frequently. BTW I adjusted the system preferences, so that the keyboard focus moves between all controls. (see http://www.tipstrs.com/tip/1505/Tab-key-to-select-form-elements-in-Firefox-on-the-Mac). But I don't think the problem is related to that.