My 'upgrade' to Yosemite on my MacBook Air (mid-2011) seems to have gone off without a hitch, though I hardly do heavy-duty stuff on it, nor have I any experience with the whole iOS 8 interconnectivity thing - still using my iPhone 4 and its running 7.1.2. I'm hardly overwhelmed by Yosemite - maybe just whelmed - as it hasn't added anything new functionality or obvious stability to my computer that I felt it needed for my use. Meanwhile the OS now sits at 10.10 - essentially point 'improvements' over an OS 13 years old - the last couple apparently tied to the Ive'ization of the desktop and the increased desperate integration into the Apple 'environment'. Take that for what it seems.
I use both Safari and Firefox with Yosemite. As ever, Safari seems to use less resources and memory than Firefox - but its not a browser I enjoy, preferring Firefox (and primarily its usability add-ons) as my Default. And Firefox recently updated itself to v.33.1, for what that's worth.
IIABDFI (If It Ain't Broke....)