Hi

If you follow this support article to add a custom search engine:

https://support.mozilla.org/en-US/kb/manage-my-default-search-engines-firefox-android#w_add-a-search-engine

and use this as the search string:

https://www.perplexity.ai/?q=%s

...you will have Perplexity as the search engine in Firefox for Android.

adding a full custom search engine seems to be disabled by default. Found the solution in a forum that explains how to open the possibility to declare any engine in the search engines of Firefox : https://superuser.com/questions/7327/how-to-add-a-custom-search-engine-to-firefox/1756774#1756774

Hoping that Mozilla will add this (very usefull) option in the parameters soon ...