Currently (v1.3.6, Firefox 42, Mac OS X 10.11.1) the last entry in the userFilters text field is easily hidden behind the horizontal scroll bar if:

a) some filter is longer than the screen is wide, and
b) one mouses over the text field.

Please consider implementing something to automatically append an empty line to prevent this little UI problem.

Long example filter (too long on 1280x800 screen"): spaceweather.com##div > table > tbody > tr > td:nth-of-type(1) > table:nth-of-type(3) > tbody > tr > td:nth-of-type(2) > table > tbody > tr:nth-of-type(2) > td > table > tbody > tr > td:nth-of-type(4) > div:nth-of-type(1) > form > div