All Travis CI builds of new PRs are failing

[OwenEdwards]

Description

All Travis CI builds of new PRs are failing, even ones which don't change the codebase (see #5602 and https://travis-ci.org/videojs/video.js/builds/456148811?utm_source=github_status&utm_medium=notification).

Error output (From Travis CI)

Running "shell:karma-run" (shell) task
> video.js@7.3.0 karma-run /home/travis/build/videojs/video.js
> karma start test/karma.conf.js
16 11 2018 20:43:24.762:INFO [framework.detect-browsers]: Detecting browsers is disabled. The browsers of the browsers array are used.
16 11 2018 20:43:35.409:INFO [framework.browserify]: bundle built
16 11 2018 20:43:35.432:INFO [karma-server]: Karma v3.1.1 server started at http://0.0.0.0:9999/test/
16 11 2018 20:43:35.432:INFO [launcher]: Launching browsers travisFirefox, travisChrome with concurrency unlimited
16 11 2018 20:43:35.437:INFO [launcher]: Starting browser FirefoxHeadless
16 11 2018 20:43:35.454:INFO [launcher]: Starting browser ChromeHeadless
16 11 2018 20:43:36.127:INFO [HeadlessChrome 0.0.0 (Linux 0.0.0)]: Connected on socket Bx06whqXUwgXJnYxAAAA with id 26409926
16 11 2018 20:43:36.153:WARN [middleware:karma]: Invalid file type, defaulting to js. browserify
16 11 2018 20:43:37.213:INFO [Firefox 63.0.0 (Linux 0.0.0)]: Connected on socket UI5h0oVzBDG8gFZIAAAB with id 72587728
16 11 2018 20:43:37.236:WARN [middleware:karma]: Invalid file type, defaulting to js. browserify
................................................................................
................................................................................
....................................16 11 2018 20:43:41.113:WARN [middleware:karma]: Invalid file type, defaulting to js. browserify
............................................
................................................................................
................................................................................
........................................................16 11 2018 20:43:43.915:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:43.927:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:43.941:WARN [web-server]: 404: /test/relative-three.mp4?test=test
........................
............16 11 2018 20:43:44.966:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:44.969:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:44.986:WARN [web-server]: 404: /test/relative-three.mp4?test=test
...............16 11 2018 20:43:45.825:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:45.837:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:45.852:WARN [web-server]: 404: /test/relative-three.mp4?test=test
.....................................................
..............16 11 2018 20:43:47.320:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:47.326:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:47.344:WARN [web-server]: 404: /test/relative-three.mp4?test=test
..............................................16 11 2018 20:43:48.829:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:48.844:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:48.865:WARN [web-server]: 404: /test/relative-three.mp4?test=test
...............16 11 2018 20:43:50.026:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:50.042:WARN [web-server]: 404: /test/relative-one.mp4
16 11 2018 20:43:50.062:WARN [web-server]: 404: /test/relative-three.mp4?test=test
.....
..................................................
Firefox 63.0.0 (Linux 0.0.0) sourceset > source before player > video el data-setup one source FAILED
	Test took longer than 5000ms; test timed out.
	internalStop/config.timeout<@node_modules/qunit/qunit/qunit.js:2001:7
	
	
	no additional sourcesets
	Expected: 1
	Actual: 0
	setupAfterEach/</<@/tmp/test/unit/sourceset.test.js:100:6 <- /tmp/b8434a11eaa2c8ceb00366e4914ba30e.browserify:49274:7
	
	
	no source set on dispose
	Expected: 1
	Actual: 0
	setupAfterEach/</<@/tmp/test/unit/sourceset.test.js:103:6 <- /tmp/b8434a11eaa2c8ceb00366e4914ba30e.browserify:49278:7

[gkatsev]

So, after two days of trial and error since we can't repro locally, we realized that we never ran firefox in travis before and are unsure as to why tests are failing there. We'll disable firefox again in travis for PRs and maybe schedule some time to investigate this issue. Though, ultimately, we probably should resurrect PAM and the twbs/savage project to safely run PR tests in Browserstack.