Logged out when accessing multiple sites within subdirs on one domain from multiple tabs in one browser.

(I have tried to search for similar issues here but found nothing relevant, not through googling for site:drupal.org either)

Two different users (through the same ISP, havent verified outside yet) starts simultaneously to experience that they are logged out of the other sites when logging into Drupal sites in another tab in the same browser. (using English Firefox 1.5.0.3)

The sites in question are a mix of 4.6.x and 4.7.0 sites, all installed and accessed through separate subfolders within the same main domain. These are not using subdomains or their own domains.

No updates or changes have been done on the computers, but two of the Drupal sites have just been updated to Drupal 4.7.

Can the reason be in some new session handling in Drupal 4.7?
- if so, is there a way around this? Using sub-domains might solve it (might also depend on the browser?), but want to hear if someone knows what is causing this before digging further into this issue.

Any advice/pointers is appreciated.