At this point, probably because everyone else uses. I did look into it when Google started throwing their weight around (also when Google decided that the solution to the memory usage problem was to suspend all tabs but the visible one and then reload them when you switch, making browsing extremely annoying and making me lose my place on pages; I ended up installing an extension to stop this, but it means I now need to be careful with Facebook, as Google also changed things to allow a site to max out the computer's RAM).
I actually have a Firefox extension (Auto Tab Discard) that enables this behavior on Firefox, although it only does so when a tab is inactive for a set amount of time, and when reloaded the view remains the same.
...and I use the ability to block JavaScript on a per-site basis to get around sites that block adblockers or incognito browsing (usually paywalled news sites on the latter), which Firefox lacks.
This is also doable on Firefox with an extension (YesScript2). It allows you to "semi-block" a page as well, which blocks some subset of JavaScript features but not all of them.
I've never run into a site that seemed to know I was browsing with Firefox's Private Browsing mode, nor can I imagine a way a site could detect such a thing. I suspect Chrome sends some signal communicating such to the site, probably because the Chrome devs see making a Web browser as a means to provide additional opportunities for capitalism, while Firefox devs see it as a means to provide additional opportunities for communication. Looking at it from the Firefox perspective, whether or not you are in Private Browsing mode is none of the website's business.
Plus the font difference makes many sites, including my own, look weird and IMO less aesthetically pleasing than on Chromium-based browsers.
I am pretty much going to experience font differences no matter what, as the "web-safe" font set (Arial, Times New Roman, Verdana, Trebuchet, et al) is completely absent on Linux, and these font names are aliased to other fonts with equivalent metrics (any specification of "Arial" is aliased to "Liberation Sans", for instance). Even then, I notice periodic variations based on underlying infrastructure changes in what I can only assume is the X server or the window manager—there seem to be two versions of the DejaVu font package that I use as the default sans-serif font when none is specified by stylesheet, and which one I see swaps back and forth from time to time when I upgrade system versions.
Font rendering and choices are starting to matter less and less now that websites can provide a Web font and ask the browser to render it in that. You can choose whatever font you want and not have to worry about whether the user has it installed or not. This is made easier by Google hosting a wide array of these font files that can then be specified in CSS by simply copy-pasting from the Google Fonts page.
1. There's no way to warn "this is a download, do you want to do it". The only way to do so (which I've enabled) is for it to ask where you want your download to go and then click cancel if I don't want to download it. Fortunately, despite the number of webpages saying "download PDF", viewing them doesn't require downloading.
This is an issue with Chromium-based browsers too. It's annoying that there's no way to say "I don't want to keep this, just directly open the cached copy instead of saving it permanently" - especially when a site forces you to download something that should be viewable in-browser, like an image or PDF.
The way that Firefox handles it, at least on Linux, is to save the file to a cache directory in systemland somewhere (mine is
/tmp/mozilla_scott0/—my Linux username is
scott so I'm guessing the 0 is to separate multiple Firefox profiles; I'm guessing it would use a subdirectory of its install directory on Windows) and then periodically clean up the directory after files are no longer being used.
It really depends if the site has a viewer or not, too. With our SharePoint sites, clicking a PDF in any browser will allow the document to be viewed in the browser.
I think it's pretty silly to provide an embedded PDF viewer in a Web page anyway. I have my own PDF viewer preferences, let me use those instead of enduring your reinvention of the wheel cause you think it's nifty-looking!