I regularly translate Thai to English.
Firefox Translate will then flip the order to Translate English to Thai when i next open an English language page (assumedly fully trusting page metadata that the site is English, without parsing page content to validate that assumption).
This is not a logical sequence. 99% of the time, when i click Translate on an English page, it is because part of the page is Thai and I want to translate that to English.
Retain last-used combination at least for the duration of the current session. Don't action any "smarts" to change the order.
I suspect this logic came from norms in Translate apps where flipping languages is common. This does not apply to the scenario of web translation in the majority of cases. The flip language logic comes from the more common use-case of translation apps - conversational translation. Web translation is primarily one-way.