How to start in safe mode

Yep.

Yes you can...

Browse to where you installed FirefoxPortable. Usually
[x]:\PortableApps\FirefoxPortable

[x] being the your drive letter.

Go into the "Others" Folder and then the folder labeled "Source".

From there.... Copy "FirefoxPortable.ini" (To be sure, Right click on it, and Select Properties, See if it says if it's an INI file to be sure you've got the right file).

Copy that file over to The Folder where FirefoxPortable.exe is....

Open up the file.

And type this in as the AdditionalParameters value.

-safe-mode

Unfortuanely you will have to do that every time you want to start it in safe mode.
And when you don't... Delete -safe-mode from the AdditonalParameters="VALUE"

You can leave FirefoxPortable.ini where it is.

So the contents of the INI file should be...

[FirefoxPortable]
FirefoxDirectory=App\firefox
ProfileDirectory=Data\profile
SettingsDirectory=Data\settings
PluginsDirectory=Data\plugins
FirefoxExecutable=firefox.exe
AdditionalParameters=-safe-mode
LocalHomepage=
DisableSplashScreen=false
AllowMultipleInstances=false
DisableIntelligentStart=false
SkipCompregFix=false
RunLocally=false

# The above options are explained in the included readme.txt
# This INI file is an example only and is not used unless it is placed as described in the included readme.txt

Another method would be:

Start->Run-> x:\portableapps\firefox\firefoxportable -safe-mode

just make sure your path takes you to the FFP directory

Tim