Firefox 46.0.1 not supported on Mountain Lion

Unless you previously downloaded Mavericks or Yosemite, they are no longer available. Your option will be El Capitan.

Check to make sure your applications are compatible.

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

Do a backup before installing, preferable 2 backups on 2 different drives.

One option is to create a new partition (~30- 50 GB), install the new OS, and ‘test drive’ it. If you like/don’t like it it, you can then remove the partition. Do a backup before you do anything. By doing this, if you don’t like it you won’t have to go though the revert process.

Open Disk Utility, select your hard drive (step 1), then the Partition tab (step 2), and select the partition. Using the /// at the bottom move it up (step 3) until the size box decrease by about 50 GB. Select the newly created space and hit the + button (step 4). Name it something and select Mac OS Extended (Journaled) as the format (step 5). Then hit the Apply button(step 6). Download the installer from the App Store and when it starts, point it at the new partition. You might want to make a copy of the installer outside the Applications folder to avoid having to re-download it in the future. Once installed, go to System Preferences/Startup Disk, select the new partition and reboot. Test away.

Alternate web browsers still supporting older OS X versions.

SeaMonkey

http://www.seamonkey-project.org/

Waterfox

https://www.waterfoxproject.org/

FF for 10.8 (+ 10.6 and 10.7) will be good until FF 49, to be released in August. After that, switch to 45 ESR (the ESR gets security patches, but no new "features"--most of which recently have been complete garbage, so nothing lost), which will be good through mid-2017. After that, you need to upgrade the OS, or run a different FF based browser.

I am just trying out Waterfox 44.0.2, which is an EXACT clone of Firefox 46.0.1 which I am currently using (Waterfox also updates automatically). When I ran Waterfox the first time, my complete Firefox profile was there (extensions, themes, plugins, bookmarks etc.), but Waterfox seems to load websites much quicker. I sent an email to the Waterfox creator who assures me that support for Waterfox on Mountain Lion shall continue after July 2017, here is his reply on the Waterfox forum:

“I don't see any reason why not. Usually Mozilla end support so if users have issues they don't have to waste resources trying to fix them. Mozilla technically don't support XP 64-Bit, but I've had no issues allowing Waterfox to run on legacy systems. I'll give it a go for you, it's just as easy as changing the version number in info.plist. If anything crops up though it might be difficult to fix”.

So Waterfox 44.0.2 is now my main browser with Safari as my backup. This, of course, is just my experience. I am not much of a techie but Waterfox, as long as it works well, seems to be the answer for me, Ken.

For now Waterfox is probably OK, if it means simply porting over some FF version. However, there are big changes in the works at Mozilla. They will be implementing Web Extensions API, which will replace the current user interface language XUL, which is what addon developers have been using. This will mean all addons will have to be re-written from the ground up when this hits, or begins to phase in, most likely in FF 48. Many developers will simply walk away from their addons, leaving them unsupported and/or extinct. This new Web Extensions API will also severely restrict what an extension can do. From what I can see the addon developer community, and many "loyal," until now, Firefox users, are mostly completely enraged and bewildered by this. In addition, also in the works is "Electrolysis" or "e10s." All of this a lame attempt to "modernize" Firefox by fixing what isn't broken.

Mozilla claims all this will be beneficial, as it will allow FF extensions to be cross-browser compatible (meaning to Chrome.) Rather stupidly, in order to gain market share against Chrome, Mozilla seems intent on trying to turn FF into a Chrome wannabe--but what's the point of a Chrome wannabe when you can just use Chrome? This will leave by the wayside the superior customizability of FF (by way of themes, addons) which is what drew me to Firefox in the first place. Another thing is that a new rendering engine "Servo" is in the works, as well--it's still in development, so not sure when that hits, or what FF will look like when it does.

So what I'm getting at is I wonder what the developer of Waterfox will do when all this arrives, and won't simply be able to port over the current FF version? I hope I'm wrong, but I strongly suspect that he will just have to give up. And extremely important, I also wonder if said developer will be able to keep up with security patches, as needed, now and for the future.

And what's the point of using Waterfox now, when you can continue with Firefox 45 ESR until mid-2017? If Waterfox stays in development by then, then at that point switch over. I'm using the 45 ESR, which will keep getting needed security patches until its end-of-life (at Firefox 52, I believe.)

Mozilla has been in a death spiral since the horrible Australis UI was rolled out. For many, the only thing that prevented them from leaving FF completely (myself included) was/has been the addon Classic Theme Restorer.

Now, if even CTR stops working (not to forget NoScript), or becomes mostly a shadow of its former self when this new garbage arrives, that's the end of the line.

I appreciate the concerns you express WZZZ about Waterfox, but the creator of it is very young, very savvy so I'll wait to see what happens.I am not much of a techie, but as a 64 bit browser, Waterfox runs much quicker, more reliably than Firefox, therefore a better FF experience IMO. I am sure there will be many new browsers until FF stops supporting 10.8. If nothing works by then, I can always use Safari that came with 10.8. That's why, contrary to other opinions, I also run both Sophos for Mac Free Edition + Anti-malware Bytes, just my choice.

I'll check out FF 45 ESR as well, thanks for that suggestion. Ken.

...I can always use Safari that came with 10.8

It's no longer safe to use. Not getting patched for security vulnerabilities. Not getting any more support from Apple since last Fall.

I hope Waterfox will be a viable Firefox alternative, once Firefox completely self destructs. But for the reasons I mentioned, I'm not holding my breath.