User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:62) Gecko/201100101 Firefox 62.0 Build ID: 20181001155545 [Affected versions]: - Firefox 62.0.3 - Firefox 63.0 - dev edition - Firefox 62.0a1 - Nightly [Affected platforms]: - Ubuntu 18.0 and 16.04 [Steps to reproduce]: 1. Navigate to about:preferences and scroll down to Applications. 2. Locate PDF and change the Action. 3. Select Use other... and select a different Fx version than the one already opened. ( e.g if you have 62 Release opened, select DevEdition or Nightly). **Note: This action is not working on Windows, as it does not change the browser to another version ( only to other browsers like IE or Chrome). 4. Search for a sample PDF and select it. [Actual result]: Upon selection, an infinite number of tabs keep opening. I have attached a short video of the issue. [Expected result]: The PDF is saved and opened with the selected browser version.

Upon further research it would seem that this issue is already reported. Marking as Resolved: Duplicate.