How to change behavior of ctrl+t in Firefox to open new tabs next to current tab

As of Firefox 61 this behavior is available natively:

1. Navigate to about:config
2. Find the browser.tabs.insertAfterCurrent row
3. Toggle it to true by double-clicking the row

You could try Tab Mix Plus, an add-on I've been using since forever-ish.

https://addons.mozilla.org/en-us/firefox/addon/tab-mix-plus/

Once installed, in the Tab Mix Plus options, Events -> New Tabs -> "Open new tabs next to current one". The option below it, "Open other tabs next to current one", doesn't do exactly what you're looking for.

Here is a relevant bug report:

Bug 528005 - Provide ways of opening new tabs next to the current tab

Here are some more suggested extensions:

• Always Right
• Open Tabs Next to Current
• Open Tabs Next to Current Plus

Personally I'm using the second one.

I'm using the Tab Control addon for the similar reason.

https://addons.mozilla.org/en-us/firefox/addon/tab-control/

There is a setting: Position new tabs to right of current tab.