Descriptive Problem Summary:
In 507.1253 and 507.1254, Firefox will no longer finish loading the webclient. It'll die after downloading the resources. Checking my console shows this error as the last output:

"Uncaught Error: NoSuchMethodError: Cannot call "removeRule" (a.removeRule is not a function)" webclient.dart.js:11404
"Stack Trace:
.zC.PU@http://ip:port/res/webclient.dart.js:9690:18
.UB@http://ip:port/res/webclient.dart.js:450:1
.jV.$1<@http://ip:port/res/webclient.dart.js:2034:1
.yv@http://ip:port/res/webclient.dart.js:7307:13
.rq.$0@http://ip:port/res/webclient.dart.js:7705:1
.vs.static.HZ@http://ip:port/res/webclient.dart.js:7657:24
.vs.R8@http://ip:port/res/webclient.dart.js:7588:1
.pZ.$0<@http://ip:port/res/webclient.dart.js:7693:16
.OM.Ki@http://ip:port/res/webclient.dart.js:7792:15
.Cx@http://ip:port/res/webclient.dart.js:7240:16
.BG<@http://ip:port/res/webclient.dart.js:7244:5
.Av.$0<@http://ip:port/res/webclient.dart.js:6041:1
.dr.$0@http://ip:port/res/webclient.dart.js:6739:15
.aX.vV@http://ip:port/res/webclient.dart.js:5641:5
.zd@http://ip:port/res/webclient.dart.js:5386:22
.ft<@http://ip:port/res/webclient.dart.js:6245:13
.tR/z</<@http://ip:port/res/webclient.dart.js:6255:46

"

The webclient still seems to work fine in Chrome.

If I drop back down to version 507.1252, it works in both Chrome and Firefox.

Numbered Steps to Reproduce Problem:
1) Host a game from Dream Daemon in 507.1254
2) Connect via the webclient in Firefox
3) Resources download and then the webclient dies
4) Check the JavaScript console for the error
5) Connect via Chrome and the webclient runs fine

Expected Results:
Firefox can run the webclient

Actual Results:
The webclient dies in Firefox with the error "NoSuchMethodError: Cannot call "removeRule""

Does the problem occur:
Every time? Or how often? Every time
In other games? Tried a test project and an old game
In other user accounts? On administrator
On other computers? Unknown

When does the problem NOT occur?
When using Chrome, or when using 507.1252.

Did the problem NOT occur in any earlier versions? If so, what was the last version that worked? (Visit http://www.byond.com/download/build to download old versions for testing.)
The problem does not occur back in 507.1252

Workarounds:
Use chrome.