Sometimes it can happen that the MetaMask wallet in the Firefox browser shows an error: MetaMask had trouble starting 
Sometimes it’s enough to press the 
Restart MetaMask button, but unfortunately, sometimes it doesn’t help.
Next, you can try clearing your browser’s cache and cookies or disable other extensions… and if none of these options helped and you don’t have your seed phrase or private keys saved, then this exact guide is for you 
.
I’ll show you where to find these MetaMask vault data in Firefox’s folders. And how to decrypt the vault data to get your seed phrase and private keys back.
Alright, let’s get started
You need to know the password for your MetaMask 
wallet. That’s obvious.
Ways to Find Your Firefox Profile Location in Windows:
Type about:support into your browser.
When you open a Firefox Profile folder: 
xxxxxxxx.default-release
Open folder storage 
and in the storage folder 
open the default 
folder.
Find a MetaMask 
Internal UUID
Open Firefox and type about:debugging#/runtime/this-firefox in the address bar.
Now i know that my MetaMask folders have a name: feaa69fe-435c-4dc8-8f73-9941fd189f56 your files will have another name.
Open a folder with a longer name
moz-extension+++feaa69fe-435c-4dc8-8f73-9941fd189f56^userContextId=4294967295
In this folder search file with number. My file is 709 this file contains MetaMask vault data.
Download and install EmEditor: https://www.emeditor.com/#download
Or use your favorite text editor, preferably one with more features 
Right-click on your file and open it in the program EmEditor
Search for words like: data, salt or iterations, algorithm
Text editing 
You have to find something like this:
{“data”:“cWPVjHP0PiCEov5EPWuGI7kobFSAoMqKC4CyX5hmPEufIGTyFP3BkugW5x79eVJlA594IOMczHYtesLwpdbvYKHcFQtdpdJT1RDhO4fJH6S3Le54IlWNGwLxuccl6CaLZjEg1CtMp/+LsCFPBSndcVhku7bP6UETXoM0dxRVwE”,“iv”:“Eor1W8DJPU90MorZvqGw8w==”,“keyMetadata”:{“algorithm”:“PBKDF2”,“params”:{“iterations”:600000}},“salt”:“nYSGJv+aTm6w6lyP81IEMC+EdHsuj0Ojp10HNITSZRk=”}
Sometimes the word ‘data’ at the beginning may appear like this: {“!ôë”:“cWPVjHP…” and you need to add the word data manually.
Download Vault Decryptor:
https://github.com/MetaMask/vault-decryptor and unzip this file to the your desktop 
Open folder vault-decryptor-master -► and open index.html file in the browser
Paste the {“data”:} and use the password you set for your wallet in the MetaMask extension and click the “Decrypt” button as shown in the video by MetaMask - Randy
Everything is done and now you see your MetaMask seed phrase
If my guide helped you, you can support my always‑hungry cat or hedgehog 
My MetaMask ETH address is 0x8A00f43C099bEB3F5d3C289e8b93c71c32B4d52e
PS: I will update this guide in the future because a few things have changed 
I’ll add a few tricks.