Unsure what information is needed to (or even where I should) bug report,, but there doesn't seem to be a contact website/email available under "More Information" on this add-on's page. :(
On Firefox 57.0.1 (64-bit) for Windows 7, running Mind the Time 2.2.0, Mind the Time doesn't show anything on the summary page. The top navigation bar (MIND THE TIME, ..., REFRESH) does show up, and so does a color gradient along the entire page (below the navigation bar). Mousing along the navigation bar does "dark-shade" the box behind the text, and clicking on any of the words doesn't do anything--the exception is "ABOUT" which opens this page.
Mind the Time shows up fine as an icon on the Toolbar; even the h:mm time tracker increases as it should. When clicked, the popup menu showing the various timer settings shows up fine; the h:mm/h:mm time tracker (showing time spent on the current domain) also works fine.
The bug persists, even after restarting firefox, and even after turning off all other addons and running the default theme.
edit: Refreshing firefox (completely new install) fixes the problem, somehow!