Apparently this isn't a Firefox issue.
https://www.mediawiki.org/w/index.php?title=Topic:Suspcq0bf5nd3gsd&topic_showPostId=sv4ojjy1c4aa0q7o#flow-post-sv4ojjy1c4aa0q7o
Firefox Win7. New account: Default VE.
Firefox Win7. IP edit: Default VE.
Chrome Win7. New account: Default VE.
Chrome Win7. IP edit: Default VE.

Unfortunately, it's impossible to determine from that brief list whether this comment means "I didn't ever see a prefs dialog" or "It's working correctly, since I always started in the editing environment that test2 is configured to offer first".

IE8 test: locked out completely. All that comes up when I create a new account and click EDIT is the link to report problems.
On EnWiki the wikitext editor comes up fine.

@Whatamidoing-WMF: I get both dialogs, one after the other, same in Chrome.

I'm running Firefox 41 on Mac OS 10.10.5. I'm not seeing the second dialog (as of 10 seconds before starting to type this reply). Are you testing this on a Mac?

No, not on a Mac.

I can confirm what @Whatamidoing-WMF reported with FF 42.0 on Mac OS 10.11.2. I get the "Start editing" screen, but not the choice dialog.

If I look into the JS console, I see the following, which may or may not be related:

19:37:26.203 [Edit] Missing or empty schema1 load.php:37:129
<anonymous> load.php:37
handler() load.php:155
jQuery.Callbacks/fire() load.php:45
jQuery.Callbacks/self.fireWith() load.php:46
jQuery.Callbacks/self.fire() load.php:46
mw.track() load.php:155
self.prepare() load.php:35
self.logEvent() load.php:36
handleTrackedEvent/<() load.php:33
jQuery.Callbacks/fire() load.php:45
jQuery.Callbacks/self.fireWith() load.php:46
.Deferred/</deferred[tuple[0]]() load.php:47
handlePending() load.php:159
execute/runScript/markModuleReady() load.php:162
execute/runScript/<() load.php:162
jQuery.Callbacks/fire() load.php:45
jQuery.Callbacks/self.add() load.php:45
.Deferred/promise.always() load.php:46
execute/runScript() load.php:162
execute/</checkCssHandles() load.php:163
execute() load.php:163
mw.loader.implement() load.php:170
<anonymous> load.php:1

19:37:27.810 Exception in store-localstorage-update:1 load.php:177:493

19:37:27.811 NS_ERROR_DOM_QUOTA_REACHED: Persistent storage maximum size reached DOMException [NS_ERROR_DOM_QUOTA_REACHED: "Persistent storage maximum size reached"
code: 1014
nsresult: 0x805303f6
location: https://test2.wikipedia.org/w/load.php?debug=false&lang=en&modules=jquery%2Cmediawiki&only=scripts&skin=vector&version=%2Ba17lhsh:175]1 load.php:177:531
log() load.php:177
handler() load.php:155
jQuery.Callbacks/fire() load.php:45
jQuery.Callbacks/self.fireWith() load.php:46
jQuery.Callbacks/self.fire() load.php:46
mw.track() load.php:155
flushWrites() load.php:175
triggerIdle() load.php:178

I thought this might be caused by a call to localStorage.setItem (your local storage is full), but both of ours are in try blocks. I doubt it's related to Mac vs. other operating systems.
@Whatamidoing-WMF: Do you see anything in JS console when you reproduce this issue?

On Windows laptop both with Firefox and with Chrome created a new editor and didn't see the big preference dialog. This isn't a mac or ff exclusive issue.

I believe that the dialog is only shown to a user if all of the below are true:

• they're logged in
• they haven't disabled VE somehow (via preferences)
• their tab preference is set to the default (remember last)
• they've edited before a specific date/time (set by server configuration, currently defaulting to 1st January 2016)
• they've never seen this dialog before (based on hidden preference)