Extensions that are enabled should still be enabled when you close and restart Firefox. Do you lose other data or settings when you restart Firefox?

You can try "Clear the startup cache..." in "Help -> More Troubleshooting Information" (about:support).

• https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox

I have no problem with extensions. It's dictionaries I'm having trouble with.

My locale is Greek, and so is the built-in dictionary. I've added an English as well, since I'm mostly typing in English.

As I'm now composing my response to you, every single word is highlighted since only the Greek dictionary is enabled by default. Enabling the English dictionary from the context menu (right click in the frame -> Languages -> check the extra dictionary) fixes this, but I have to do this step on each and every new tab I open!

I would like to have this extra dictionary always enabled, automatically on each FF startup and for all tabs opened.

Clearing the startup cache did not help unfortunately.

My issue seems to be the same one as discussed in https://support.mozilla.org/en-US/questions/1228800, I'll continue discussion there.

You should have multiple dictionaries always enabled, there's no need to enable English in every new tab. So your case doesn't look normal, even though you're using Firefox 115 ESR. Somehow this setting is not remembered.

I'd try to add English to a list of preferred languages.

1. Click the menu button ≡ and select Settings.
2. In the General panel, scroll down to Language under Language and appearance.
3. Click the Choose... button next to Choose your preferred language for displaying pages to see your list of languages.

The first language setting in Settings is about the language for the user interface. The second language setting in Settings is about the preferred languages for webpages.

• Settings -> General -> Language and Appearance -> Language
• https://support.mozilla.org/en-US/kb/use-firefox-another-language
• https://support.mozilla.org/en-US/kb/choose-display-languages-multilingual-web-pages

Installed language packs will appear under "Tools -> Add-ons -> Languages" (Ctrl+Shift+A). Installed spell checking dictionaries will appear under "Tools -> Add-ons -> Dictionaries".

• Settings -> General: Language and Appearance: "Check your spelling as you type"
• https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

The first language setting in Settings is about the language for the user interface. The second language setting in Settings is about the preferred languages for webpages.

• Settings -> General -> Language and Appearance -> Language
• https://support.mozilla.org/en-US/kb/use-firefox-another-language
• https://support.mozilla.org/en-US/kb/choose-display-languages-multilingual-web-pages

I took a while to respond, apologies for that. Made a lots of tests and somehow made that thing work. BTW, the dictionary I was trying to use is a combined/bilingual one: https://addons.mozilla.org/el/firefox/addon/english-greek-spelling-diction/ , see that other thread for the advice you've given there.

After I've read your last response above, I've tinkered a bit. Specifically,

1. I've add English as a secondary user interface as per your instructions. Doing so, added also a new dictionary ("English (US) Dictionary") to the two dictionaries I've had had
2. Visiting a page in English I can now see that the dictionaries I've had are now both enabled, and the US one is disabled! That's just what I've needed to work.

However, I've decided to check what would happen if I removed the English US environment altogether, which I had a bit of trouble to find how it's done, since even though the article https://support.mozilla.org/en-US/kb/use-firefox-another-language explains how to remove a language, it did not work; in the list of firefox language settings I was unable to press remove when selecting "English", the button was grayed out (possible bug on 115 ESR?). I found that I could accomplish that by going to extensions (ctrl+shift+a) and there was a special category at the left, whereas I could select english and remove it.

Removing the UI English language, did not also remove the English (US) dictionary that got installed with it.

I then checked again whether spell-checking on English pages was enabled after firefox restart, using the English-Greek dictionary. When I first tested this it kept the setting enabled for the bilingual dict! But redoing the same procedure on another rig failed.

As a final test, I removed the English (US) dictionary that got installed with the English (US) firefox UI and restarted. This time it always failed.

I know this sounds like a lot of mumbo-jumbo here to be able to make the combined dictionary work, but that's my experience. And it's very strange I've had had to enable an English user interface in order for this to work.

TL;DR: a combined Greek-English dictionary https://addons.mozilla.org/el/firefox/addon/english-greek-spelling-diction/ on my Greek locale Firefox could not be automatically enabled on FF start. Solved it as follows:

1. Added English (US) Firefox user interface as per https://support.mozilla.org/en-US/kb/use-firefox-another-language#w_add-languages-to-the-firefox-interface but made sure that my own language was the first one (to keep FF interface in my language). Restart
2. Start Firefox, press CTRL+SHIFT+A to go to the extensions panel and select the "Languages" category at the left. Delete the English (US) language and restart
3. Make sure that you do not delete the English (US) dictionary!! Additionally, enable your own extra dictionary. Now the setting will stick.
4. If it does not work, uninstall the bilingual dictionary and reinstall it and re-enable it. This time the setting should stick.