I find this extension extremely useful, but it always hangs on site https://msdn.microsoft.com/vba/office-vba-reference. For most pages in the site, I regularly get the message "A script in the extension "Legibility" is causing Firefox to slow down". The interruptions continue even after I uncheck "Enable" in the Legibility pop-up, and the only way to browse without disturbance is to disable the extension under about:addons. To re-create the problem, navigate to the above website then click random entries in the left-hand column under "Table of Contents". The extension misbehaves only on these Microsoft pages.