I was a heavy user of rikaichan and also rikaisama. Must admit, I always neglected the Anki-integration in any of these addons (as I'm not using Anki) but instead focused on the "dictionary" functionality.

As both, rikaichan and -sama have had problems for a long time now, I was forced to reduce my usage of these addons. I updated to Firefox 55 today, marking both addons as legacy. Because both are no longer in development, I decided to finally remove them as there's no point anymore.
Just by coincidence I found this addon and it does essentially all I ever wanted. It works absolutely great and is more than just a replacement.

I have no idea how I missed out on this addon. I did a bit of googling and queries like "rikaichan alternative" are not turning up this addon at all. I hope it will get picked up by search engines eventually and the word spreads further.

The functionality of this addon is top notch. For people who came here searching for a rikaichan/sama replacement: you found it.

There are two changes in "design" from rikaichan I want to highlight/mention:
a) Scanning key. It doesn't process all text under your mouse constantly but rather requires a push-to-talk-like key. I actually wished for this in rikaichan already, so I welcome this change.
b) The dict results are unified while in rikaichan you toggled/switched the result pages of each installed dictionary. I must admit, I'm not yet sure what's better as both approaches have their merits.
Yomichan's unified output shows you all the information in one view, that's great for an overview of all the stuff. Also the kanji integration/kanji information presentation is way superior to rikaichan's "switch to kanji-dict result page"-approach.
However, yomichan unifies all dictionaries (except kanji) and while there's a point in e.g. unifying JMDICT and KireiCake results, I wouldn't want JMnedict results "in the same view" as I don't see them contextually fit well with each other. Granted, the illustrated example should be rather rare.
I also enjoyed in rikaichan having the English JMDICT results as first page and my native language's JMDICT results on a sub-page because occasionally I liked to check how they translated stuff into my native language. But that's something I rarely did and probably not a widely needed use case. I assume in yomichan multiple languages would also get unified but I did not test that yet.

I have yet to check how yomichan works on HiDPI displays as rikaichan/sama acted up on these (incorrect positioning of the popup, i.e. not at the highlighted words). It should work though, as I'd assume webextension technology transparently handles such stuff.

Overall, yomichan is imo not just an alternative to rikaichan but more like a successor. More than rikaisama ever was.