PFF Non-friendly extensions

Anything write-intensive. And you can usually tell by the nature of an extension (what it does) whether it will write alot. Anything that saves your session, for instance, would need to write to the drive every single time you move between pages. This is the reason history is disabled by default as well... to save on writes (you can always enable it in Options). There are simply too many extensions to keep track of -- not to mention how much they sometimes change between versions -- to maintain a solid list of what will cause issues, so you just have to use your judgement.

This applies to other programs as well, not just Portable Firefox. Anything write-intensive will slow down everything you're doing from your drive.

Write-intensive extensions aren't a big deal at all from a portable hard drive (the regular-size or notebook-size ones) or from high-speed flash drives (not "USB 2.0 high speed" -- they all say that -- ones that actually use high-quality flash ram and controllers like the lexar lightning and some of the OCZ drives).