This only seems to happen using FF version 36 or above. Running the following command:

Actions.InvokeScript("Ext.ComponentQuery.query('form-fieldtypes')[0].query('textfield[name=textfield1]')[0].setValue('ARHL0000046');");

Using this ExtJs demo page: http://dev.sencha.com/extjs/5.1.0/examples/kitchensink/?locale=en&theme=neptune-touch#form-fieldtypes

Will result in this error:

Overall Result: Fail
------------------------------------------------------------
'5/25/2015 2:35:34 PM' - Executing test: 'Scratch HTML Test', path: 'Scratch HTML Test.tstest.'
'5/25/2015 2:35:34 PM' - Using .Net Runtime version: '4.0.30319.34209' for test execution. Build version is '2015.1.515.0'.
'5/25/2015 2:35:34 PM' - Starting execution....
'5/25/2015 2:35:36 PM' - Detected custom code in test. Locating test assembly: TestExamplesProject.dll.
'5/25/2015 2:35:36 PM' - Assembly Found: E:\Users\Dad\Documents\Test Studio Projects\TestExamplesProject\bin\TestExamplesProject.dll
'5/25/2015 2:35:36 PM' - Loading code class: 'TestExamplesProject.Scratch_HTML_Test'.
------------------------------------------------------------
------------------------------------------------------------
'5/25/2015 2:35:36 PM' - Using 'FireFox' version '38.0.1' as default browser. 
'5/25/2015 2:35:36 PM' - 'Pass' : 1. Navigate to : 'http://dev.sencha.com/extjs/5.1.0/examples/kitchensink/?locale=en&theme=neptune-touch#form-fieldtypes'
'5/25/2015 2:35:36 PM' - 'NotRun' : 2. Wait for '2500' msec.
'5/25/2015 2:35:40 PM' - 'Pass' : 3. Wait for Exists 'TextFieldSpan'
'5/25/2015 2:35:40 PM' - 'Fail' : 4. [Scratch_HTML_Test_CodedStep] : New Coded Step
------------------------------------------------------------
Failure Information: 
~~~~~~~~~~~~~~~
Exception thrown executing coded step: '[Scratch_HTML_Test_CodedStep] : New Coded Step'.
InnerException:
ArtOfTest.WebAii.Exceptions.ExecuteCommandException: ExecuteCommand failed!
InError set by the client. Client Error:
mozCommandProcessor: InternalError: too much recursion
BrowserCommand (Type:'Action',Info:'NotSet',Action:'InvokeJsFunction',Target:'ElementId (tagName: '',occurrenceIndex: '-1')',Data:'Ext.ComponentQuery.query('form-fieldtypes')[0].query('textfield[name=textfield1]')[0].setValue('ARHL0000046');',ClientId:'37319b90-a514-4a60-9e59-1017e66c482a',HasFrames:'False',FramesInfo:'',TargetFrameIndex:'-1',InError:'True',Response:'mozCommandProcessor: InternalError: too much recursion')
InnerException: none.

   at ArtOfTest.WebAii.Core.Browser.ExecuteCommandInternal(BrowserCommand request)
   at ArtOfTest.WebAii.Core.Browser.ExecuteCommand(BrowserCommand request, Boolean performDomRefresh, Boolean waitUntilReady)
   at ArtOfTest.WebAii.Core.Browser.ExecuteCommand(BrowserCommand request)
   at ArtOfTest.WebAii.Core.Actions.InvokeScript(String script)
   at TestExamplesProject.Scratch_HTML_Test.Scratch_HTML_Test_CodedStep() in e:\Users\Dad\Documents\Test Studio Projects\TestExamplesProject\Scratch HTML Test.tstest.cs:line 52
------------------------------------------------------------
'5/25/2015 2:35:41 PM' - Detected a failure. Step is marked 'ContinueOnFailure=False' aborting test execution.
------------------------------------------------------------
'5/25/2015 2:35:41 PM' - Overall Result: Fail
'5/25/2015 2:35:41 PM' - Duration: [0 min: 4 sec: 461 msec]
------------------------------------------------------------
'5/25/2015 2:35:44 PM' - Test completed!

NOTE: Running the exact same JavaScript using the FF console works as expected. It's only a problem trying to run it using Test Studio Actions.InvokeScript API.

Attached is a sample test that reproduces the problem.