This severe review follows:
1. Years of usage of Context Search < 6
2. 3 weeks of usage of Context Search 6
3. A few days of usage of ContextSearch-web-ext
4. 1 person-week of contributions to contextual search extensions, mostly to this one
5. Thorough consideration.

"Upgrading" to version 6 turned a satisfied user into a dissatisfied ex-user.

The long version is available in ticket #436 of ContextSearch-web-ext’s issue tracker (titled "I wish you wouldn't copy so many features from other developer's work"). If you want to spare yourself from a ½-hour read of the gory details:
1. This extension is proprietary and developed by a single person, who calls himself a hobbyist.
2. It is unclear if that person’s distribution of this software is legal.
3. This extension brings significant value, but in exchange for important upfront investment (time spent installing, configuring and mastering, not to mention maintenance).
4. The 215 issue reports against this extension were just made private.

If you just thought there would be a Firefox extension for that and figured you would be up and running 10 minutes later, you will be disappointed. If you think contextual search can eventually save you 1 person-hour, then I recommend looking at these extensions first:
1. ContextSearch-web-ext
2. Swift Selection Search (essentially abandoned in 2021)
3. Context Search Origin

If none of these work or satisfy you, then consider trying this one.

Going forward, there is one big advantage this extension has over all the above though: support for WebExtensions manifest version 3. Mozilla intends to keep supporting version 2 for the foreseeable future (with an advance notice of at least 1 year), but this will probably mean that in the short term, this extension will become cross-browser (available for Chromium), unlike the above.

Updates:
1. ContextSearch-web-ext now (1.48) supports WebExtensions manifest version 3!
2. The ITS’s owner made it public again.