In FPR18, Reader mode has two main changes: first, it is updated to the same release used in current versions of Firefox (I rewrote the glue module in TenFourFox so that current releases could be used unmodified, which helps maintainability), and second, Reader mode is now allowed on most web pages instead of only on ones Readability thinks it can render. By avoiding a page scan this makes the browser a teensy bit faster, but it also means that edge-case web pages that could still usefully display in Reader mode now can do so. When Reader mode can be enabled, a little "open book" icon appears in the address bar. Click that and it will turn orange and the page will switch to Reader mode. Click it again to return to the prior version of the page. Certain sites don't work well with this approach and are automatically filtered; we use the same list as Firefox. If you want the old method where the browser would scan the page first before offering reader mode, switch tenfourfox.reader.force-enable to false and reload the tab, and please mention what it was doing inappropriately so it can be investigated.
Reader mode isn't seamless, and in fairness wasn't designed to be. The most noticeable discontinuity is if you click a link within a Reader mode page, it renders that link in the regular browser (requiring you to re-enter Reader mode if you want to stay there), which kind of sucks for multipage documents. I'm considering a tweak to it such that you stay in Reader mode in a tab until you exit it but I don't know how well this would work and it would certainly alter the functionality of many pages. Post your thoughts in the comments. I might consider something like this for FPR19.
Besides the usual security updates, FPR18 also makes a minor compatibility fix to the browser and improves the comprehensiveness of removing browser data for privacy reasons. More work needs to be done on this because of currently missing APIs, but this first pass in FPR18 is a safe and easy improvement. As this is the first "fast four week" release, it will become live January 6.
Merry Christmas to those of you who celebrate it, and a happy holiday to all.