There a preference for the visited link color that you can edit using about:config, but on sites that set their own colors, this won't have any effect by itself. Here are the steps:

Part I: Get the hex code

If you want to use the same color as your desktop version of Firefox, you can find the there this way:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste visit and pause while the list is filtered

(3) Make a note of the value for the browser.visited_color preference, which should be a # sign followed by six characters in the range of 0-9, A-F

Part II: Modify your setting on Android

(1) Open a new tab to about:config (type about:config in the Awesomebar and tap the go arrow or tap Enter).

You may want to bookmark this for easier future access.

(2) In the search box, type visit and pause while Firefox filters the list

(3) Scroll down to browser.visited_color and tap the preference, then tap next to the end of the existing value and edit it to the new one, then tap away from the control to save.

If you switch back to your other tab and test on a site that doesn't override colors, you should see the change.

Success?

Thank you for the reply. I tried the option and it seems to work on some sites as you mentioned. On other sites, it does not work, for example google.com.

Is there a way to enforce this setting across all sites? On desktop there is an addon called Visited Link. Is there any similar solutions?

Thanks

Most likely there is a setting to override website color choices on Android as there is on desktop, but on desktop, this can lead to not seeing any background colors or images, so it's not a very useful setting.

I don't know whether there is anything like Stylish on mobile to apply custom style rules to websites (i.e., override style rules on websites) but that would be a more tailored way to address it if there's an add-on like that.

Also, Google may be something of a special case if it is using tracking links in the results instead of direct links to the site. On desktop, the tracking is added when you click the link; I haven't checked on mobile.

Thanks for your replies.

What I'm trying to point out is: changing visited link color is not really an issue on Firefox on PC, and in Chrome on Android, the google seach results results are colored differently for visited links. So why can't I do the same thing in Firefox on Android?