Using Firefox 3.5.6, Vista 32 home premium SP2. I'm logged in on the main site (http://slashdot.org), but whenever I open a story up - whether in a new tab or the same window - I'm logged out. Using the login link at the upper right takes me to my profile page - click through to the home page, where I'm still logged in - try going back to the story - logged out again. Lather, rinse, repeat.
No issues with this when using Chrome.