Long waits for Firefox and SeaMonkey to respond to links from other applications

When I open a web address in (for example) Thunderbird, there's a wait 
of around fifteen seconds before the web browser, already open, handles 
the address.

Affected browsers:

- Firefox
- SeaMonkey
- Waterfox.

Not affected:

- New Moon (Pale Moon) – the waiting period is a split-second
- Chromium – split-second
- Falkon – less than two seconds.

Any ideas?

<https://put.re/player?id=b62zHf8k.m4v> shows the waiting period with 
Konsole as the starting point, Firefox as the default web browser.

I can't say exactly when the problem began, but it was long before 
Firefox 66.

----

$ date ; uname -v
Sun 24 Mar 2019 16:23:22 GMT
FreeBSD 13.0-CURRENT r345330 GENERIC-NODEBUG
$ pkg query '%o %v %R' firefox seamonkey waterfox
www/firefox 66.0_3,1 poudriere
www/seamonkey 2.49.4_24 FreeBSD
www/waterfox 56.2.7.2 poudriere
$