Further to my earlier mention of a temporary Opera fix I have developed a marginally better solution.
Use these instructions to
create a User JavaScript folder if you do not already have one.
Once that has been done create a text file with the name 'wincusto-fixes.js' (minus quotes naturally) and paste the following code into it:
// ==UserScript==
// @include http://*.wincustomize.com/*
// ==/UserScript==
opera.defineMagicVariable('m_Opera', function (value) { return false; }, null );
opera.defineMagicVariable('m_Moz', function (value) { return true; }, null );
opera.defineMagicVariable('browserID', function (value) { return 3; }, null );
You should now be able to remove any site-specific preferences masking your browser as Firefox; the advantage being that now any server-based statistics will be able to correctly count Opera ensuring it doesn't disappear from the stats altogether. (A common catch-22 being a site not supporting Opera due to a low count in the stats, when the count is lower than it should be due to visitors being forced to use another browser or hide Opera's identity.)
Note that I have seen the pinned post and appreciate the promise to support Opera (if you want people to test who are familiar with the typical places that sites end up breaking Opera (usually inadvertently relying on browser bugs such as Gecko's faulty handling of capturing event listeners) I'm happy to lend what time I can) even if there is no ETA, hell most of what I've seen works even under stock Opera. This is just to keep things ticking over as best as possible in the meantime.