I too am on 89.0b1 and have no issues. TO start TB in Safe Mode: 1) ensure the app is closed, 2) hold down the left Shift key and start the app as normal. You'll be presented with a "Thunderbird Troubleshoot Mode" window. Choose Diable all add-ons and Continue In Troubleshoot Mode.
Bug 1707803 Comment 3 Edit History
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I too am on 89.0b1 and have no issues. To start TB in Safe Mode: 1) ensure the app is closed, 2) hold down the left Shift key and start the app as normal. You'll be presented with a "Thunderbird Troubleshoot Mode" window. Choose Diable all add-ons and Continue In Troubleshoot Mode.