When I first set up my computer, I had ATI cards in it. Those have since died on me and I'm using an Nvidia card as a replacement. Since installing the Nvidia drivers, Waterfox and ONLY Waterfox has its window contents shifted up slightly. If I uninstall/disable the drivers, the issue goes away. Another reason I know that it's just a graphical issue is that I have to adjust where I click in Waterfox down slightly to interact with it properly. If I go fullscreen with anything, (the browser window, video, browser game), it works fine again. Windows GUI elements seem to be fine, the menu drop downs work properly as well as the Save As dialog box. Anyone know how to fix this, or possible troubleshooting steps to diagnose what's going on with this?
Thanks.
Troubleshooting/Observations:
- It's not a scaling issue as that doesn't fix the problem.
- Not an addon issue since disabling them doesn't work.
- Disabling drivers fixes issues.
- Affects multiple versions of graphics cards and drivers. 700 series and 900 series. Only one used at a time.
- Only Waterfox has this issue.
- Executable profile in the Nvidia settings doesn't seem to do anything, though I may not have gotten the right settings.
- ATI drivers were uninstalled.
- Waterfox is up to date.
- Latest drivers for Nvidia don't fix it.
- Menus and other dialogs are in the proper places and are visually correct.
- History/Downloads/Bookmark window, and the bookmark update window have the same issue, so this happens to all of Waterfox.
Here's a pic of what is going on:

EDIT: Disabling hardware acceleration resolves the issue. Disabling WebGL in the config options does not though.
EDIT 2:
Using dirdi's answer, I was able to find an option to disable using OpenGL with Waterfox. It's using DirectX now, and I have no black bar. This issue seems to affect both Firefox, (Waterfox is just a rebranded, old GUI Firefox), and Chrome. The Driver WSI Info listed in about:support all state that Google is the vendor for the WebGL drivers, so it's potentially something wrong in their driver. I've seen that Chrome can also have this issue with Hardware Acceleration enabled at times. What lead me to find this was the following in about:support
OPENGL_COMPOSITING
disabled by default: Disabled by default
available by user: Enabled via layers.prefer-opengl
What's kinda odd is that it was set to true, and I set it to false, so I don't know if I enabled it or disabled it.