The look-and-feel should change less than the range of available "addons" - small offerings by Mozilla community developers of tweaks and modifications of the vanilla version of the browsers.
The move is to make addon development parallel in large measure to the development environment existing for Google's Chrome browser. Firefox has steadily been losing market share to Chrome. They must be worried, and being largely a volunteer effort, supported as to permanent employees by donor contributions and partnership subsidy [i.e., the default browser search function had been Google and there was a switch to Yahoo, accompanied, presumably by a bidding war or some other offering or chastisement from one of the larger players or the other].
Consider the dilemma if you offer a browser with configuration options to block advertisements and tracking cookies effectively, and Google's infusing cash and its business model is search and apps for free in exchange for user profiling and selling of generic info for targeted advertisements that advertisers/merchants pay, to reach what might be a more receptive audience than would be reached by a firm opting for generic vs. targeted ad buying.
The Duck-Duck-Go search engine, suggesting it is more amenable to user privacy than Google, uses examples such as you search Google for "herpes," and med insurers and big Pharma suppliers get profile information from Google in exchange for giving Google money; consider potential consequeces.
Duck-Duck-Go is a search provider alleging it does NOT track usage, so it is unclear where it gets its money.
Aside from that, as Google's Chrome browser attains greater market share at the expense of Mozilla, with Microsoft IE and its new EDGE browser being a third place option for default users, the addon/extension community follows market share, and so Mozilla believes it must accord that fact attention by making it easier for developers who have done Chrome extensions [where the market share return on effort is greatest] to port their product to Firefox.
Interested readers might consider this link. The big major Firefox addons for privacy and security are the ad-blockers, and NoScript which is a product/addon that edits out a lot of the scripting websites add on beyond what's needed in a clean design to get content to a web user, and to beyond that, via scripting, to do profiling and third party tracking via permanent cookies downloaded to a user's machine, etc., so that profiling can be done by the firm itself without having to give Google (or Microsoft, now, with its unique advertising ID baked, now, into Windows 10) money in exchange for what it might mine on its own.
Like it or love it, Microsoft is looking to eat Google's lunch from its near-monopoly stranglehold on the Operating System by moving the sort of mischief Google does into the OS, and bless you, you get a "free" upgrade.
Some readers might care about all this, most would not, but I care about it and choose what I post.
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And to my Republican friends, try sometime a "Google" = Facebook privacy tracking
It might prove informative.