First, when trying to save a profile, it would be truncated at ~90K due to hitting assertions saving extension data. Then after effectively removing the code within the extensions block (such that an empty block was saved), when parsing the 'meta' section of a profile from Fennec, perf-html.io barfed on the lack of a valid "extensions" entry; with pain I was able to figure out a valid-looking entry to provide in order to get the profile to load. I'll attach the patch I used to get the profile to save with an empty extensions block.

I just tried to profile Fennec after the recent fixes and everything seems to work with an extension (Privacy Badger) installed on Fennec. Resolving now, but please reopen if this somes up again.