Set up the test environment

Download the latest Beta from https://beta.mozilla.org/
Run installer and select customize
Put the install in a known directory (C:\FirefoxBeta for purposes of this example)
Do NOT allow it to launch Firefox at the end of the install

Open C:\FirefoxBeta\defaults\pref\channel-prefs.js in a text editor
Change the line reading pref("app.update.channel", "beta"); to pref("app.update.channel", "foobar");
Save and Close the text file

Run cmd.exe
Change directory to the folder where you installed Firefox: cd C:\FirefoxBeta
Run firefox.exe: firefox.exe -P -no-remote - This will bring up our profile selector window.
Use the "Create Profile" button to create a profile new profile
Once the profile is created, select it, and then click "Start Firefox"

Test

1. Start with a new profile (see above)

2. Resize the window so it's at least 1024x768

3. Go to nytimes.com, scroll up and down for ~15 seconds

4. Open another tab and go to pinterest.com, scroll up and down for ~15 seconds.

5. Switch back and forth between the two tabs three times

6. In nytimes.com tab, go to youtube.com and play this video, this video, and some other random video for 15 seconds

Note: If a video plays with Flash find another video to test. To check, right mouse click on the video and choose “Stats for nerds”. The video is valid if the mime type is video/mp4 or video/webm.

7. Go to cnn.com, scroll for ~15 seconds then find a video and play it

8. Update your graphics driver to the latest available version

Note: don't restart Firefox or your computer if it can be avoided. If Firefox crashes, report the crash, restart Firefox and continue on to step 9.

9. Repeat the test from step 3 through 7.

Note: if Firefox crashes, report the crash, restart Firefox and repeat steps 3 through 7.

10. Quit Firefox after all steps have been completed.

Fill out this form once you've completed the test (one response per system please).

Report bugs for the following issues:

You can use this template to file your bug report.

be sure to attach a copy of your about:support and dxdiag log in the bug report
if the bug is a crash, be sure to include links to your crash reports from the about:crashes page