Some additional test results. I was installing Gnome extensions and ran into bizzare problems
For on extension, using FF version 54. I stayed on that page, within a minute or two the system would lock up. I think that the page was presenting new images, without returning memory to FF.
Following good advice on how to preserve SSD life, I formatted the SSD with btrfs. Actually any file system that does not do journalling would be useful. BTRFS does COW which reduces the I/O to the SSD when compared to ext4.
And some Gnome extensions seem to fail if the host file system is btrfs
I've contacted the author, gave lots of examples, and am now awaiting a response.
I am not sure that this problem is acceptable as a F26 bug . With F25, I was am not using btrfs.