The next solution helped me to increase the startup time.
- Start FireFox
- Type about:config in the address bar
- Search for the preference name browser.sessionstore.enabled
- Edit the value to false
- If you cannot find it, you should create this preference name:
1. Right click somewhere in the window
2. Choose New>Boolean
3. Enter the preference name browser.sessionstore.enabled
4. Select the value false in the 'Enter Boolean value' window
5. Press 'OK'
6. Restart FireFox
with this value set to this preference, you will not have the option for session restore if FireFox crashes. To get the session restore option back, just edit this preference value to true or just delete this entry (Right click on preference and select Reset, restart FireFox and your entry is gone).
Search also for browser.sessionstore.resume_from_crash and set this value also to false.