Follow the instructions here:

https://support.mozilla.org/en-US/kb/pop-blocker-settings-exceptions-troubleshooting

I normally add QBO to an exception list for Firefox when setting up on […]

Resolution for 'clients machines issue: 'How do I unblock popups on Mozilla Firefox browser to allow QuickBooks access.?' available: Yes.

The clients machines issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the clients machines issue yourself by looking for a resolution described in the article. If it is a complex clients machines issue or you are unable to solve the issue, you may contact us by clicking here or by using other clients machines support options.

For more information about resolving issues related to clients machines in Quickbooks/QBO or to get support for the issue 'How do I unblock popups on Mozilla Firefox browser to allow QuickBooks access.?', click the link below:

Details