After looking more closely, I can see when this is about to happen. When the time bar starts turning grey to indicate that a portion of the video is loaded, I notice just before the video freezes the portion of the video preloaded will disappear and then immediately jump back.

This also seems to happen between two and three minutes into a video.

  Sometimes I can fix this by clicking on the time bar a little ways back and it'll fix itself. More often, I have to reload the page whereupon the video restarts from the beginning. 

You're lucky the only thing you have to do is reload. Ubuntu itself freezes up on occasion. I've found out it was because (maybe, maybe not) I had to install some proprietary driver for my graphics card.

Firefox has hardware acceleration set to "on" by default, but it's still young, which means some video cards or drivers might have problems with it. You can try to disable hardware acceleration to see if that alleviates your problems.

You might need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.).

Then perform these steps for Firefox version 29.0 and above:

• Click the menu button and select Options for Windows or Preferences on Mac or Linux. For Firefox 28.0 and previous versions, click the orange Firefox button at the top left, then select the "Options" button, or, if there is no Firefox button at the top, go to Tools > Options.
• In the Firefox options window, click the Advanced tab, then select "General".
• In the settings list, you should find the Use hardware acceleration when available checkbox. Uncheck this checkbox.
• Now, restart Firefox and see if the problems persist.

Additionally, please check for updates for your graphics driver by following the steps mentioned in the following Knowledge base articles:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

• Upgrade your graphics drivers to use hardware acceleration and WebGL

Disabling hardware acceleration seems to have solved it. Shame because videos seem to perform a little worse now.

The problem seems to be occurring again. Disabling hardware acceleration does not seem to have permanently solved the problem.