Φόρουμ υποστήριξης - Firefox

What can I do to prevent websites from logging me out "due to inactivity" and "for your safety" (lol)? This happens with many of my financial (banking/brokerage) sites, for instance, or some tax return sites.

I went them to be like my email accounts (e.g. gmail, yahoo mail). You can have a tab open for Gmail and another tab open for yahoo mail, and even if you don't click or type a single character in hours, they will not log me out.

Is there some way to trick these patronizing "We'll log you out for your good" websites from logging me off?