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How do you start Portable FireFox in safe mode?
It's just a switch. -safe-mode You can start FirefoxPortable.exe from the commandline with it and it'll pass it to firefox.exe.
Sometimes, the impossible can become possible, if you're awesome!
When I asked you this question about the U3 version of FFP you included a profile paramater (something like "-profile ..\). Is this not needed in regular FFP?
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Things have got to get better, they can't get worse, or can they?
In Firefox Portable, it's really similar to Firefox. So, whereas with Firefox, you start it with firefox.exe -safe-mode ... in Firefox Portable you start it with FirefoxPortable.exe -safe-mode. It's more difficult to do this in FF for U3 due to its ties to U3, requiring an initital launch in non-safe mode to update paths followed by a more-complex commandline to Firefox itself.
If U3 could pass a command line parameter to FirefoxForU3Start.exe (one of the four "launchers" needed to make Firefox for U3 work), then we could do something similar. But, being a consumer-oriented platform, U3 doesn't support things like command line parameters.