Hi,
F-droid is a good start! You can find all that you need there for what you want to do (“I will use the phone for SMS/MMS, internet, photo, agenda, music play”).
If you really want to install Firefox, you could try this app that should help you download firefox: FFUpdater | F-Droid - Free and Open Source Android App Repository
But there are other options out there on F-droid. For example, Lightning works very well:
Lightning | F-Droid - Free and Open Source Android App Repository
There is a thread about browsers, maybe you can get inspiration:
For the rest, you don’t really seem to need Yalp. There already are preinstalled apps on FOopenOS for what you want to do. You may want to change the camera app to open camera, the agenda app to etar (or something else), etc. Just check what is available on F-droid.