I find myself concerned about the increased memory footprint. IMHO, Firefox uses too much memory as it is, so a 10%-20% increase is a step in the wrong direction. I know that Chrome uses more memory, but that doesn't make Firefox any better.
Also, the third-party addons aren't going to be changed my Mozilla. On the contrary, Mozilla is simply going to replace its entire addon API, leaving all pre-exsiting addons in the dust. The addon developers will be expected to either port their addons to the upcoming WebExtensions API, or get left behind. The developer of one of my favorite addons, DownThemAll, has express his/her concerns about the future of DownThemAll, as well as Firefox addons in general.
"The new APIs would only allow for a severely limited in functionality, severely stripped down DownThemAll! at best", says an article posted to the DownThemAll website. The article continues:
"Gone with DownThemAll! will be add-ons that e.g. let you change major bits about the Firefox user interface (e.g. tabs tree add-ons), add-ons that allow you to do more â€œadvancedâ€ stuff than just showing or slightly altering websites, such as e.g. restarting the browser upon click (unless mozilla kindly provides an API for that, which wonâ€™t be compatible with Chrome, of course). Add-ons like NoScript will be severely limited in their feature set as well."
Hopefully, the new WebExtensions API won't be as limited as the developer fears. Even so, given the number of available addons inevitably being rendered useless, it could take years for rewrites and/or replacements to repopulate the addon ecosystem.
According to Mozilla, part of the reason for switching APIs is to avoid issues with electrolosis. The current API wasn't designed for a multi-process architecture, meaning that many addons will be broken anyway. This could be the reason Firefox is still single-process to this day, even though others are multi-process.
On a more positive note, more security is certaily a welcome benefit. Also, I've noticed on may occasions the entire browser slowing down while one or two tabs render a page, with one processor core doing plenty of heavy lifting, and the others sitting idle. As pointed out in the article, multiple processes can help with that. If one process crashes, it can be restarted by another. So, I guess it's a mixed blessing.