Where is that WMS capability? (What is WMS?) Zoom in on an area you’d like to see a WMS overlay for — say, a chunk of Canada. Then go to Add > Image Overlay. Select the Refresh tab, then click on WMS Parameters. This brings up a pop-up window that allows for some sophisticated fine-tuning of WMS content:
There are plenty of WMS servers preloaded for you to try right out of the box:
I chose a server with content for Canada, chose a few layers, and I got this:
In this particular case, every time I pause, the WMS server will be queried anew for an area approximately the size of the screen. Seamless.
[Update 10:19 UTC: Forgot to mention the coolest part: Once you have WMS-based image overlay, you can of course just right-click on it in the Places sidebar and save it or send it along to others. They in turn don’t have to know anything about WMS to be able to enjoy the same functionality you’ve just created.]