Firefox can import passwords from a CSV file, but the import feature may not be enabled. This is because it is a relatively new feature and was first made available in Firefox version 80.
There seem to be performance issues when this feature is turned on. For best web browsing experience, turn the feature on, import your passwords, and then turn the feature back off.
Follow these steps to enable the Firefox file import feature.
Start a Firefox web browser on your computer.
To open Firefox advanced configuration preferences:
Don't worry about the "Proceed with Caution" warning. After you enable this feature and import your passwords to Firefox, you will disable the feature again.
You are now in the Firefox advanced configuration preferences section. To enable import from a file feature:
Type signon.management.page.fileImport.enabled in the search box (Step 1).
Then click on the toggle button to turn it on (Step 2).
Check that the import from a file feature is now enabled. It should be bold and set to true.
signon.management.page.fileImport.enabled = true
Now that you turned on the Firefox feature that enables importing passwords from a file, go back to Step 3 in the import passwords to Firefox guide.