Backport (Firefox 67) downgrade protection to Waterfox Classic

[grahamperrin]

Waterfox Classic

https://developer.mozilla.org/docs/Mozilla/Command_Line_Options#User_Profile

Firefox 67's Downgrade Protection prevents accidentally starting Firefox in a profile running a later version of Firefox. Depending on changes between the two versions, some files in a profile may not be downwards compatible. Adding this option bypasses Downgrade Protection.

Without comparable protection in Waterfox Classic:

– with the generically-titled Waterfox Profile Manager to the left, it's too easy for an end user to inadvertently misuse a Waterfox Current profile with Waterfox Classic.

Debatably less of an easy pitfall with the about:profiles view of things.

Critically: I overlooked the Profile Manager use case at #1241 (comment) where (a few hours ago) I wrote:

end users of Waterfox Classic are not likely to fall into this trap; a Waterfox Current profile must be explicitly chosen

Also, it was wrong of me to assume that end users will implicitly associate 68 with Waterfox Current.

Waterfox Current (beta/pre-release)

There may be the temptation to begin using Waterfox-Current- (in lieu of 68-edition-) as a default prefix to visible profile names for Waterfox Current.

In any case there's nothing in the name of a profile to hint that it should be avoided when using either Waterfox Profile Manager or Waterfox Profile Manager. (Nothing, unless the user chooses to include a phrase such as "don't use with Classic" when using Waterfox Current to create a profile … and that ain't gonna happen.)

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Meta, tracking: #538, #582