Admin page doesn't work with Firefox, only IE8

[pacman10]

I'm running SnippetMaster PRO and want to add a User.

I've read the instructions, i.e.

   1.  Log into SnippetMaster with a user that has admin privileges.
   2. Click the admin link. (Located at the top right area of the page.)
   3. Click the Users tab.

I've logged in successfully as admin but when I click the admin link it just takes me back to login in again and I simply go round the circle. Nowhere is there a tab marked Users.

P.S. Have discovered the source of the problem: the SnippetMaster admin page doesn't work with (my) Firefox. If I use IE8 it works.