If someone else logs in to your account, it invalidates your current cookies, and logs you out on your next visit. This happens not only when they use a different browser, but when they login with your password, or use the "Become This User" feature.
A few ideas of what you may want to try:
1. change your password.
2. be consistent about what browser / device you use to log in with.
3. check that you dont have any other user who can use the "Become This User" feature, such as other admins.
