#768: [CI] Fix Firefox on macOS screenshot to be full page

It works! https://455-598343-gh.circle-artifacts.com/0/tmp/circle-artifacts.fXfLmPS/imgs/Firefox_54.0_%2528evergreen%2529_OS_X_10.11_15.6.0.png

Fixes #720
diff --git a/script/screenshots.js b/script/screenshots.js
index 63d9374..547669b 100644
--- a/script/screenshots.js
+++ b/script/screenshots.js
@@ -116,8 +116,8 @@
     .spread(function(scrollHeight, viewportHeight) {
       console.log(sessionName, 'get scrollHeight, clientHeight', scrollHeight, viewportHeight);
 
-      // the easy case: Firefox and IE return a screenshot of the entire webpage
-      if (cfg.browserName === 'Firefox' || cfg.browserName === 'Internet Explorer') {
+      // the easy case: IE and Firefox on Linux return a screenshot of the entire webpage
+      if (cfg.browserName === 'Internet Explorer'|| (cfg.browserName === 'Firefox' && cfg.platform === 'Linux')) {
         return browserDriver.saveScreenshot(baseDir + '/imgs/' + fileName + '.png')
         .then(willLog(sessionName, 'saveScreenshot'))
       // the hard case: for Chrome, Safari, and Edge, scroll through the page and