As a user it is important that you keep your private key save. It is strongly recommended to make a backup to be able to restore files, e.g. in case you reinstalled the browser.
Note that the key is saved in your browser profile, so make sure your home folder is encrypted and no one has access to it.
Check your keys
You should regularly check the keys on your system.
You should also avoid having too many different keys, in case one is compromised.
When uploading, always check for how many users the file is shared, and take note of the message telling you for how many keys the file will be encrypted:
If the number of keys does NOT match up, make sure to notify the administrator immediately as this might possibly be an incursion attempt.
Private Key Access
The E2EE app allows various way how to handle the private key. The public key is public and stored on the cloud server - if you send files only the public key is required. The private key comes into play only when you want to open an encrypted file. The private key is never sent to the server.