When you say "all the current extensions will be gone" what do you mean exactly?

Every extension ever made for Firefox over the last 13 years won't work with Firefox II (or New Firefox) (think New Coke or Coke II) ; every extension will need to be re-written from scratch to work in Firefox II and the vast majority won't work anywhere the same as they work now. Just as 'extensions' or add-ons made for Chrome or Opera won't work in Firefox, the non-XUL based version of the upcoming "new" Firefox will require a completely new type of 'extension' - the current XUL based extension code won't work. And the new API's will greatly limit what extension developers can and can't do.

And a few long time Firefox extension developers whose extensions [that I am using now] work just fine currently [who haven't written a new extension for many years] have already stated that they don't plan to support the new add-on infrastructure - i.e.; waste their time re-writing their existing extensions for Firefox II.

You have to keep in mind that many extension developers start when they are in uni / college, maybe as part of their curricula, and once they get into the 'real world' other things usually become a higher priority in their life. Then they stop writing new extensions and only do minor compatibility updates to what they already contributed the the world-wide Mozilla "community". And it's not like asking for contributions for an extension provides even a part-time renumeration for their time - I have done email with one (soon to be former) extension developer who only asked for "beer money" and for his efforts he didn't even get enough for the first beer per coding session.

As far as another browser that will be like Firefox or have the extensibility of the past and current Firefox goes, look to SeaMonkey or PaleMoon right now. And I wouldn't be surprised if another 2 or 3 Gecko based browsers are already in the initial stages of start-up right now as a result of The Future of Developing Firefox Add-ons blog. I would run out of fingers and have to take off my shoes & socks to count the number of 3rd party "Firefox" browsers I have seen and tried-out over the last 13 years; SM and PM are the only ones around now that still have ongoing, active development,

A "chem-spill" Firefox 43.0.1 version was released yesterday to fix a to meet a new Microsoft signing requirement - security related. https://www.mozilla.org/en-US/firefox/43.0.1/releasenotes/

As far as "signing" issues with Firefox 43.0.1 - why just use the available pref - xpinstall.signatures.required - and get some relief for at least the next 5 1/2 weeks until you have no choice - for real - with Firefox 44. Or switch to the ESR version and get "relief" until ESR goes EOL in June 2016. But keep in mind that some extensions may not work in the ESR version; some extension developers have already done a minVersion above Fx 38 to like FX 40 or 41.