Screenshots included right here, didn't upload earlier.

Another screenshot.

Hmm, that's strange!

When did this start, was it after an update to Firefox or after an update to Windows?

Is the checkbox present but invisible? In other words, if you click that area, does it check change the on/off status of an invisible box? For example, on the second screenshot, clicking to the left of any of either All or DDR-333 should change the list. (Actually, clicking the text should also do it because it is set up as a "label" for the control which activates the control when you click it.)

Any difference if you change the zoom level, e.g., hold down Ctrl and tap the + key to enlarge?

Could you try disabling Firefox from using hardware acceleration? This is a potential workaround for incompatibilities between Firefox and your graphics card/chipset driver software. This article describes where to make that change: Firefox's performance settings. This will take effect the next time you exit Firefox and start it up again. Any difference?

Since hardware acceleration improves the appearance of fonts and animations, you may want to check whether graphics card/chipset driver updates are available for your system. This article suggests safe ways to do that: Upgrade your graphics drivers to use hardware acceleration and WebGL.

You may have switched on caret browsing.

• http://kb.mozillazine.org/accessibility.browsewithcaret

You can toggle caret browsing off/on by pressing F7 (Mac: fn + F7).

• Options/Preferences -> General: Browsing
  remove checkmark: [ ] "Always use the cursor keys to navigate within pages"

Note that this is a Firefox accessibility feature.

• https://support.mozilla.org/en-US/kb/accessibility-features-firefox-make-firefox-and-we