Ah yes, right, the gecko engine is another piece in the UA puzzle, fetching a bit from here and a bit from there and... :-)
UA's sure are interesting, one of the most important KM-features for me. Especially since the KM engine is so old now and some stupid websites pretend it were the browsers fault if they don't show right. Funnily most of those sites do work fine after simply lying to them about the used browser
Oh well, or sometimes it's just handy to switch to the mobile version of a website, which are much less cluttered, much faster, and mostly don't insist on javascript. Or quite often I don't see the enlarged product images on amazon, then I switch UA to operamini and see at least the mid-size version of the pics. Amazon is funny anyway, looks like they use different code for such stuff like images, possibly depending on creating time of a page?! On some pages the pics-enlarging works without tweaking, on some they work with UA Opera9, on some only with a fake mobile UA. I'd be lost without UA's :-)
And sometimes all UA tweaking doesn't help, but then there's still some KM "style" macro that allows to quickly remove the CSS page layout, which often works wonders too, although the pages look extremely basic afterwards - but ultrafast to scroll, when it was nearly frozen before :-)))