After succesfully configuring the Tor connection, right after the browser opens for the first time, quickly disconnect your Internet connection
Type about:config in the address bar then press Enter
Set the following preferences to the provided values

To disable browser updates
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app.update.enabled: false
app.update.auto: false
extensions.torbutton.versioncheck_enabled: false

To disable extensions updates (Optional)
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extensions.update.enabled: false
extensions.update.autoUpdateDefault: false
extensions.update.url: ""
extensions.update.background.url: ""

Close and reopen the browser
Reconnect your Internet connection and browse normally