Either of the following link shows how:

• https://support.mozilla.com/kb/Safe+Mode
• http://kb.mozillazine.org/Safe_Mode

What kind of problems do you have that you want to start Firefox in Safe mode ?

I have no problems, thankfully. I have Firefox on another computer and somehow FF went into "safe mode." I noticed that it ran faster, with no affect on what I do with the browser, which is nothing fancy. Therefore, I decided to put FF on this computer into "safe mode," too.

I had to ask how to do it because the instructions are not evident where mention of using it to fix problems occurs under 'help.' Why talk about using something without telling how to get to it. Very frustrating! Had it been done, I would not have had to bother folks here. Perhaps bring this omission to the attention of the "powers that be."

See also Firefox is stuck in Troubleshoot Mode

Safe mode is a diagnostic mode meant for troubleshooting purposes.
So there is no need to run continuously in Safe mode.