Comment 5 for bug 257408

Nope, I see more or less what you see in Windows. The only method I've found to replicate that problem is the one I described above, and then I can resolve it either by resetting the zoom (Ctrl+0), or by unselecting "Zoom text only."

I notice that, in your Ubuntu screenshot, the page is rendered in a serif font, whereas on my computer (and on Windows, for you), it's rendered in a sans-serif font. Can you go to Firefox's preferences (in Ubuntu), content, fonts > advanced, and see what settings you have there. I've tried making my default font bigger and disabling "Allow pages to choose their own fonts", but it didn't seem to affect that page.