Hmm, I haven't heard of this before. How often are you seeing it? Some thoughts: (1) Many paths in the file are specific to your current Windows username and Firefox profile (they use absolute paths), so swapping extensions.json files between users/profiles can cause breakage. Is there any process on your computer that might be replacing the extensions.json file with an older/different version? Do you ever sign in to Windows using a different user name, or do you locate your profile on a mapped drive? (2) Has your security software reported trying to clean a threat from the extensions.json file, corrupting the format in the process? (3) Do you suspect any malware trying to force a malicious extension into the file to trick Firefox into installing it, corrupting the format in the process? For further investigation of #2 or #3, it would be useful to keep a clean backup version of file to compare with the broken version the next time this happens.