When I load a web page, it sometimes loads a blank rectangle on the right side of the page. Doesn't happen all the time, but often enough that it's quite annoying. Thought it might be trying to load a frame or something since it's always the same shape and location but it only seems to be visual. If I zoom in and out so that the rectangle is taken out of my field of view and brought back into it, it disappears. Same if I scroll down the page and bring it back up.
Started appearing when I updated to firefox 18 I think. Tried uninstalling the firefox app and reinstalling, didn't work.
Sounds like a legitimate graphics/panning/tiling related issue. You can help us by determining if this is reproducible on Firefox Beta (on Google Play) and or Nightly for Android (http://nightly.mozilla.org), and reporting back here with your findings.