Set Firefox as default browser to open links in Gmail
UPDATE:
As @Nimar points out, the method I show below is no longer required- the user can now set a browser preference directly in IOS' settings. No need anymore for the below hack- see @Nimar's Answer below-
The only way I found was through the &quot;Share&quot; facility. Basically, the procedure is to add Firefox (or a another browser of your choice) as a sharing option so it can be selected when the link in a Gmail message is long-pressed.
&quot;Share&quot; Menu Configuration:
The following illustrated procedure uses Firefox to modify the list of apps available as a source to share with to add Firefox. However, you can add Firefox as a sharing source from some other app if you care to.  Procedure will still be the same.  Please note the images are cropped to only show the parts of the interface you need to work with





Using Firefox with Gmail:
Using a Gmail message, I'll now illustrate how you can open links with Firefox using a long-press on the link:



That's how you can circumvent Google's restriction on using a browser other than their own and Apple's Safari.  Hopefully a future version of Gmail will make all this unnecessary, but until; they do...