Firefox doesn't do that on my PC (Windows 7).
If some random program activates it, I still see my previous session.
Have you considered creating an additional FF Profile (or two)?
Use one for your normal surfing and use the other for online help files, etc.
To start the FF Profile Manager
Open the Start Menu and type:
"firefox.exe -P" (in the Search box, no quotes, remember the space)
Open the Terminal and type:
"firefox -P" (no quotes, remember the space)
If you want to be able to choose profiles when you start FF remember to uncheck "Don't ask at startup".
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