missing buttons, icons, scrollbar handles in Firefox
 

(on slackware64-current, with AlienBob's KDE5)

The missing scrollbar handle problem has been solved by using NoiaScrollbars addon. But many buttons and icons are missing on some web pages. How to solve this ?

"some web pages" is too vague to investigate.

Please provide links to these pages, and at least a snapshot of one of these, with a comment to tell what is missing that you would expect to be displayed.

for example

Attachment 25275

Attachment 25276

Attachment 25277

the first with chromium, the other with firefox

this depends on your gtk+3 theme, you should use one compatible with the gtk+3 version you have installed (or just use the default provided by Slackware).
this depends just on the webpage that got broken links to images (the image it links to - right click on it -> copy image address - is simply not there anymore): it will behave the same on every browser and any operating system.

@ponce : I will try to look how to manage gtk3 themes in kde5

For the two other exemples it is true that some images are missing in the two examples (for chromium and firefox), but maybe it is because I use AdZHost (a huge mofification of /etc/hosts)
The real difference between chromium and firefox is in the second line : the box used to enter data is missing on firefox (but it is still possible to enter data)

that happens for the same reason as above: you are using a firefox version linked against gtk+3 so it still have issues with the custom gtk+3 theme you have installed.

try momentarily renaming your ~/.config/gtk-3.0/settings.ini file and login again.

@ponce : renaming ~/.config/gtk-3.0/settings.ini solved the problem. I will try to find what did create it

Apparently the breeze theming in Plasma 5 for GTK+-3 based applications is not working well for some…

If you change this in $HOME/.config/gtk-3.0/settings.ini :
to:
then things will look a lot better.