Screen distorts (shimmers, wavy lines/appearance) when scrolling in any web page.
Disabling hardware acceleration and smooth scrolling, doesn't make any difference.
Problem doesn't exist in Chromium on Xubuntu 16.04 or in Firefox 46 running on Ubuntu 14.04.
Edit: Re-installed Xubuntu 16.04 (including Firefox) and thin wavy line still experienced when scrolling, usually near the middle of the screen. Disabling hardware acceleration and smooth scrolling now makes the display usable but jerky.
Looking back, there appears to have been similar problems with various versions of Firefox and releases of Ubuntu/Xubuntu in the past, so suspect there is no solution as might be hardware (display driver) related.