Normally links open in the same tab. If you long-tap a link, you can chose to open it in a new tab.

I get that but this is android.... Every link I tap keeps coming with a new tab

You can try to clear all data of the Firefox app via Android App Manager.

Note, this would essentially erase your Firefox profile, i.e. all settings, bookmarks, etc., so you'd start from scratch. You'd also need to set up your Firefox account again on the phone in case you're using Sync to sync your bookmarks.

No what I need is help from a developer