Fix failures on tests for Firefox

[kitsonk]

Type: bug

The following has been addressed in the PR:

• There is a related issue
• All code matches the style guide
• Unit or Functional tests are included in the PR

Description:

This PR pins the version of the tests against Firefox 52, the ESR version of Firefox, which resolves the failures that can be encountered.

Fixes #138

[codecov]

Codecov Report

Merging #139 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #139   +/-   ##
=======================================
  Coverage   85.63%   85.63%           
=======================================
  Files           1        1           
  Lines         543      543           
  Branches      136      136           
=======================================
  Hits          465      465           
  Misses         32       32           
  Partials       46       46

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